The teenage Susan Sontag was lying on her living room floor, book in hand, when her stepfather walked over with a warning. "Susan," he said, "if you keep on reading so much you'll never get married." It was just after the war, a time of economic affluence and cultural complacency in America, and from the backwater that was home on a dirt road in Tucson, Arizona, there could have been little evidence that Sontag's readings of Proust would come in handy in later life, let alone be attractive to the opposite sex.

But the young Sontag could barely contain her mirth: "I just couldn't stop laughing," she says.

"I thought, 'Oh gosh, this guy's a perfect jerk. There must be millions of people out there who want to know me.' I knew there must be loads of people just like me, interested in the same stuff. Otherwise who else was writing these books and drawing these paintings, and who were they doing it for? I thought there were two worlds and if I could only get out of this one the other one would be so much more fun."

It is this combustible mixture of self-confidence, optimism and tenacity, so evident in her adolescence, that continues to drive the essayist, novelist and playwright at 69. "Sontag stands for what is articulate, independent, exploratory: for self as a work in progress," claims fiction writer, Hilary Mantel. "During her four decades as thinker and cultural commentator, as novelist, director and playwright, compliance has not been part of her brief."

"I'm very devoted to the idea of transformation," says Sontag. "It's the most American thing about me and it's what I love most about America; you're allowed to change your life, to reinvent yourself. That's what I look for in art and arts; the willingness to give it up or move on, put it in a closet or put it in storage."

And so it is, that after 20 years in storage, she has placed her essay-writing skills on display in a collection entitled Where The Stress Falls. The book gathers around 40 pieces penned between 1982 and last year - prefaces, forewords, afterwords, tributes, articles and talks - in a display case for Sontag's full range of interests and writing styles. The subject matter stretches from her love for Bunraku, a Japanese form of puppet theatre, to her account of directing Waiting For Godot in besieged Sarajevo.

The shortest is a 400-word note on Don Quixote, published in translation for the National Tourist Board of Spain, while other pieces flit from praise for individual artists, authors and filmmakers to broader themes, such as a century of Italian photography or the inherent complications in literary translation. The longest, and the one Sontag is most proud of, is on Roland Barthes: "The single most ambitious essay in the whole collection and the one that took me longest to write," she says. It is also the most emblematic.

If you're preparing for an intellectual journey through her book then you should not travel light. Her work presumes a shared breadth and depth of intellect from the reader as well as a common singularity of purpose. If dumbing-down is indeed a cerebral virus of the modern age then Sontag has had her jabs. "I guess I think I'm writing for people who are smarter than I am, because then I'll be doing something that's worth their time. I'd be very afraid to write from a position where I consciously thought I was smarter than most of my readers."

She does not read reviews: "I often feel I know what's wrong better than any reviewer does," she once said. Her friends say this is a coping strategy - a means of asserting some power over a situation over which she has no control. So she measures success principally in terms of durability. "Is it an essay that people will want to read 30 or 50 years from now, which is certainly not the case with most essays," she asks. "It works for me if it is saying things which are true, original and saying them in as eloquent, spirited, lively and lucid a way as I can."

It is the essays that have made her famous. Over the past 40 years her voice has been marked, first and foremost, by a supreme intellectual confidence, a tone evident from the first line of the first essay (Notes on Camp) that made her name in 1964: "Many things in the world have not been named. And many things that have been named, have never been described." From then on she became best known for the occasional declamatory remark: "America is founded on genocide"; "the quality of American life is an insult to the possibilities of human growth"; and "the white race is the cancer of history" are just three sparky sentences she threw into the tinder box of an America at war with the world and itself in the mid-60s. But more common was a subtle, if strident, tone that preferred complexity to simplicity and met intellectual and emotional challenges head on. Her account of a trip to Hanoi in 1968 reveals an individual both politically dedicated and personally conflicted at a time when loyalty was supposed to be determined by the camp you were in rather than the ideas that you held.

Susan Rosenblatt was born in 1933 in New York city. Her parents, Mildred and Jack, spent most of their time in Tientsin, China, where her father was a fur trader, leaving Susan, and later her younger sister, Judith, to be looked after by her Irish-American nanny Rosie, whom Sontag remembers as "an enormous, benign elephant". When Susan was five, her mother returned from China on her own, saying Jack would soon follow. A few months later, when he still had not arrived, her mother took Susan aside during lunch break and said he had died of tuberculosis. She then sent her daughter back to school.

The episode is illustrative of Sontag's emotionally spartan childhood, which produced a self-contained but not insular child. "The most meaningful relationship I had as a child was in my head. I adored my fragile, withholding mother, who was not very maternal. I had a sister who I was not close to, whom I belatedly befriended at my mother's death- bed. There was no support or encouragement. I experienced childhood as though it were a prison sentence. I never wanted to look back because there was nothing I wanted to take with me."

The family was always on the move: to New Jersey, then Florida and then Tucson, Arizona. When Susan was 12 her mother married Captain Nathan Sontag, an army air corps pilot. The children took his name, but her stepfather never formally adopted them. Sontag still refers to him as "Mr Sontag". She finally escaped at a precociously early age: at 15 she went to study for a semester at Berkeley. While waiting to register, she overheard a conversation between two older students about Proust that confirmed her arrival in a world in which she would be more comfortable. "I thought, 'that's how it's pronounced'. I'd been saying it all wrong. I thought, it's all going to be great."

The responsibilities of adulthood came on suddenly. By 17 she had married a sociology lecturer, 28-year-old Phillip Rieff. By 19, she had given birth to a son, David, who is now a journalist and to whom she remains very close. "The most meaningful relationship I've had in my life was with my child," she says, describing parenting as "an upgrading experience". They moved to Boston, where Sontag studied philosophy at Harvard and let her mind marinate in the work of the great philosophers. Herbert Marcuse stayed with them for a year. "The culture I was involved in had absolutely no relation to anything contemporary. My idea of modernity was Nietzsche's thinking about modernity."

With her late teens taken up with books, marriage and motherhood she had bypassed the traditional joys of adolescence. And it was in search of them that she divorced Phillip and headed for New York at 26, with her seven-year-old son. The end of her marriage marked the beginning of her adolescence. She says: "I had a very enjoyable adolescence from 27 to about 35, which coincided with the 60s - I enjoyed them in a way people much younger experienced them. I was practically 30 and I learned to dance. I became a dancing fool." She took a series of editing and lecturing jobs, thus starting a financially precarious and intellectually enriching career.

While she is best known for non-fiction, it is fiction that she most enjoys writing. Roger Straus, a close friend and editor at Farrar, Straus & Giroux who in 1963 published her first novel, The Benefactor, says fiction interested her most from the beginning, "perhaps, because the writers she admired most were novelists. I remember her then as very intense, very pretty and very interested in absolutely everything."

The Death Kit, her second novel released four years later, received mixed reviews and modest sales, which Straus says "she took in her stride".

But in between came Against Interpretation, which included Notes on Camp and outlined a theory of her own artistic sensibilities - railing against "interpretive criticism and mimetic art" in favour of an appreciation of artistic work for what it is rather than what we would like it to be. "Suddenly, she had a very high profile," recalls Straus. "From then on Susan Sontag was a name to be conjured with." It is from this reputation that she has long been trying to escape and why this current collection of essays has been so long coming. She says it represents a longer span of work than any of the previous collections "and the reason is that I've been trying to kick the habit of writing essays. From the beginning they received what was, in my eyes, a surprising amount of interest, which was disorienting. But it was also seductive.

"The fiction for several reasons fell to the side. I lost confidence. I knew how to write essays. I was at the top of my form and maybe at the top of the form but I didn't feel I was the best fiction writer in the world or in my country or the English-speaking world." But then came two novels which were released to widespread if not universal acclaim. First, in 1992 Volcano Lover, the fictionalised account of the triangular relationship between Lord Hamilton, his wife Emma and her lover, Horatio Nelson, and then, in 2000, In America, the tale of a Polish actress who gives up her career and resettles in California hoping to set up a commune, for which Sontag won a National Book Award. "It was only when I'd written two novels I really liked that I thought I could permit myself to collect the essays I felt were worth saving. I do forgive myself for waiting that long.

Never having had a regular job she has not had to negotiate the constraints of an institution. She has no regular working regime or timetable, no structure other than that which she imposes episodically on herself. "I don't feel the need to write every day or even every week. But when I get started on something I just sit for 18 hours and suddenly realise that I have to pee. Many days I start in the morning and suddenly it's dark and I haven't gotten up. It's very bad for the knees."

But nor has she known the security of a regular pay check. "Money was always a problem," says Straus. "It's only in the past 15 years that she's really been comfortable."

Sontag says, "It was okay because I didn't look on it as a sacrifice. I could get along without doing things I didn't want to. I was helped by a series of fellowships and grants - I had two Rockefellers, two Guggenheims and a MacArthur fellowship. But I didn't want a car or a television set, or a house in the country. I just wanted to pay the rent and make sure David had what he needed."

In her top floor flat on Paris's Left Bank, overlooking the raging high tides of the Seine, she strides purposefully through the sparse living room. She is a tall, commanding physical presence dressed in comfortable, casual black. A conversation with Sontag is a breathless event, a narrative operating under its own steam. Continuous in its logic, it careers off on endless and lengthy diversions without ever quite losing sight of the main path.

"Everything makes me think of something else," she says. "It's the story of my life." But it is nevertheless a conversation. Rare among high-profile thinkers, Sontag does not simply bombard you with anecdotes or even appear to enjoy holding court. She is interested in engaging and being engaged. You will leave with a book list ("I know you're gonna love this"), but more importantly with the impression that she would be even happier if you left her with one too. "She's a very grounded person," says actress and long-time friend Vanessa Redgrave. "Probably Sarajevo has been part of her grounding."

She loves going to the cinema where, says her friend and Italian translator, Paolo Dionardo she insists on sitting in the centre of the third row. "They are usually small art house cinemas, so it actually makes sense because you feel you're part of the film," he says.

"Susan's like a brilliant older sister," says writer and friend Darryl Pinckney. Sontag has little patience with the journalistic shorthand she believes often either distorts or disfigures meaning. She has described herself as bisexual but remains guarded about her private life. Pinckney says, "I think people get annoyed when they can't get her to talk in terms they want her to talk in. Nobody makes her use words she doesn't respect." Dionardo first met her after she had rejected the first Italian translation of Volcano Lovers, not done by him, and asked if he would do another one. He went to New York to work on it with Sontag, who reads and understands the language. "We worked on the book together word by word," he recalls. "She loves language. She can discuss the meaning of just one word for hours."

The result is a woman often depicted as formidable, arrogant and doctrinaire. Many people who knew her reputation before they actually got to know her admit feeling apprehensive about their first meeting. At the beginning of their friendship Pinckney recalls her saying: "Look, you've got to stop being scared and say what you think." She is far less intimidating than her image would suggest. True, she peppers her sentences with highbrow references, but they are always there to illustrate her point rather than illuminate her brilliance. She is sufficiently confident about her own intellect not to make you feel self-conscious about your own. She doesn't expect you to know what she knows because she presumes you know something else that she doesn't.

And alongside her intellect she has an acute emotional intelligence, equally at home talking about feelings as well as ideas, but very aware that feelings can inform ideas, and vice-versa. "I feel that my public persona is just an accumulation of misunderstandings and misperceptions, and my impulse is just to flee," she says. "How in the world would I begin to correct it?"

But while her public perception is unfair, it does not come from nowhere. Some of it stems from more awkward aspects of her personality. Even her friends describe her as "proud" and occasionally "severe". "She doesn't suffer fools, and she'll meet an argument head on," says Alan Little, a BBC reporter who met and befriended Sontag in Sarajevo during the Bosnian conflict.

A review of Where The Stress Falls in the Washington Post, accused her of a haughty writing style: "Her manner now is virtually indistinguishable from that of George Steiner in his lugubrious moments as Last Intellectual, striking that solemn pose as embodiment of high seriousness - perched atop the Nintendo ruins of western civilization," wrote Scott McLemee.

She is not averse to self-praise, saying things of herself which are true but which others might leave for someone else to point out. "I'm one of the very few essayists still in print," she says. "My collected essays have been around for 20 years and never gone out of print. They've been translated into 28 languages. That's unusual."

"She doesn't have guile," says Pinckney. "In some ways she's strikingly innocent. In this day and age of celebrity, when people want to be talked about and written about, she wants to be known for her work but not for herself. She comes from a different tradition." But much criticism stems from who she is, rather than what she has ever said or written. Some of it is undoubtedly rooted in sexism. "Being a woman is a cliche," she says. "If you are or were good looking, as I was when I was young, then it's a double cliche. Being a smart woman is just fair game. With intelligent women there is a feeling that it is inappropriate."

One of the worst things, she believes, that was ever said about her was supposed to be a compliment from Jonathan Miller, who remains a good friend. "He said in an interview that I was the 'smartest woman in America'. I just felt covered in shame and humiliation to be described in this way. First of all it's so offensive and so insulting. It so much assumes that you're doing something which is not appropriate for the category that is being named, namely being a woman. Secondly, it isn't true because it can't be true because there is no such person."

Her transatlantic lifestyle, shared between New York, Paris and, to a lesser extent, Berlin and London, seems to be born from a deeply ambivalent attitude to the US, which many American commentators understand as a sign of her aloofness. "I don't like America enough to want to live anywhere else except Manhattan," she says. "And what I like about Manhattan is that it's full of foreigners. The America I live in is the America of the cities. The rest is just drive-through."

And she is an intellectual member of what she describes as "that obsolete species - an old-fashioned liberal democrat" in a country that has little love for either liberals or intellectuals. It is a tension that propelled her into the public eye following a short article she wrote in the New Yorker following the attacks on September 11. The article questioned the use of the word "cowardly" to describe the attacks, accused commentators of infantilising the public, and ended with: "Our country is strong, we are told again and again. I for one don't find this entirely consoling. Who doubts that America is strong? But that's not all America has to be."

Sontag says: "I hope I'm not getting timid in my older years. I thought I was writing centrist, obvious mainstream commonsense. I was just saying, let's grieve together, let's not be stupid together." The reaction was ferocious; she received hate mail, death threats and calls to be stripped of her citizenship. For a few days she was part of the story. The New Republic ran an article asking what did Osama bin Laden, Saddam Hussein and Susan Sontag have in common. The answer: they all wish the destruction of America.

Sontag says: "I still think mine was the right response. But I was quite astonished. It all goes very, very deep. The American way of looking at themselves is that the US is an exception and doesn't have a destiny like other nations. Anytime anything happens in the States, people are indignant. Americans are always talking about losing their innocence, but then they always get it back again. They say 'Before, we were innocent; before, we were naive, trusting, gullible. But now we realise that it can happen here and we too are vulnerable.' My deepest fear is that this time it's true. The country does feel different. The forces of conformism and mindless acquiescence to authority have certainly been strengthened."

Friends most commonly describe her as generous - with her time, contacts, intellect and money. "When we were both living in Berlin and I had no money," recalls Pinckney, "there was a time when Susan took me to dinner 30 nights in a row." Where The Stress Falls is itself a huge act of generosity. Around half the collection is made up of tributes to other writers, artists and filmmakers. She has singlehandedly saved a number of less-well known writers from extinction by championing their work and pushing for their publication. "She's very interested in other writers," says Straus. "She brought us a number of important authors we would never have heard of."

Sontag says: "I'm very ambitious. But I don't think I'm competitive. I feel that everybody who is still doing good things is part of an informal association or company that strengthens others. One wouldn't want to be the last good writer or the last serious writer or the last serious person on the planet. You have to feel that there are people out there who are doing things that you admire and respect."

Sontag is lucky to be alive. In 1976 she was diagnosed with metastatic breast cancer. "I was supposed to be in stage four, which was terminal," she says matter-of-factly. "I sought good treatment when doctors were telling me to go on a trip and enjoy myself." She had no medical insurance, but friends had a collection so that she could find the best treatment available.

"They told me I had two years, they told David [her son] I had six months. I said, 'I just wanted to see if there was a possibility'. I sought experimental treatment here in Paris and it worked. So back in New York I'm called the miracle patient." Her response was not just medical but intellectual. In 1978 she published Illness as Metaphor, in which she argued that society has obscured and mystified its relationship to sickness by transforming it into a metaphor for social, cultural or moral decay. In what is probably her most widely read work, she used her own experience as a starting point for understanding the issue. The essay never strayed into the self-indulgent or autobiographical, but looked out to the sense of listless inertia that blights patients when what is needed is aggression and energy.

Once again she has been struck, this time by a rare form of uterine cancer, diagnosed in 1998, for which the survival rate is only 10% after five years. For several months, a couple of years ago, she was in great pain, unable to walk, and living on morphine derivatives. "A few times I seriously thought we were going to lose her," says Andrew Wylie, her literary agent. "This time, it's a different cancer," says Sontag, "but I'm in an early stage."

Facing down apparently certain death, only to confront it again in a similar form but a different place, she says, forces a permanent re-evaluation of your sense of self. "There is something about facing a mortal illness that means you never completely come back. Once you've had the death sentence, you have taken on board in a deeper way the knowledge of your own mortality. You don't stare at the sun and you don't stare at your own death either. You do gain something from these dramatic and painful experiences but you also are diminished. There's something in you that becomes permanently sad and a little bit posthumous. And there's something in you that's permanently strengthened or deepened. It's called having a life."

It was this mindset that took her to a besieged Sarajevo in the early 90s to direct Beckett's Waiting For Godot to the sound of bombing and sniper fire. Alan Little, who attended the opening night, described her presence there as being of "tremendous symbolic importance at a time when symbols really mattered. She didn't just swan in for three days and then leave. She stayed and worked." But she says few of her American friends understood her commitment.

"I would come back and my friends would say, 'How could you be in a place that could be so dangerous?' And I thought, it's okay. I didn't think I was invulnerable, because I had a couple of very close calls, and I don't think I'm a thrill-seeker. I just thought it's okay to take risks, and if ever I get to the point when I don't then take me to the glue factory."

Where The Stress Falls is published next week by Jonathan Cape at £17.99

Life at a glance: Susan Sontag

Born: January 16 1933, New York. Education: University of California, Berkeley, 1948-49; University of Chicago BA '51; Harvard University MA (English) '54, MA (philosophy) '55.

Married: Phillip Rieff 1950 (one son, David) divorced '58. Some publications (fiction): The Benefactor 1963, Death Kit '67; The Volcano Lover '92; In America 2000.

Non-fiction: Against Interpretation 1966; Trip To Hanoi '69 (contributor); Perspectives On Pornography '70; Selected Writings '76; On Photography '77; Illness As Metaphor '78; A Barthes Reader '82; A Susan Sontag Reader '82; Aids And Its Metaphors '89; Dancers On A Plane '90; Where The Stress Falls 2002.

Poetry: Under The Sign Of Saturn '80. Play: Alice In Bed '93. Films: Duet For Cannibals '70; Brother Carl '71; Promised Lands '74; Unguided Tour '83.