Everything about this book suggests it is much more the biography of a celebrity than an author. An international aristocracy of writers, artists, photographers and politicians flits through its pages; famous names – Andy Warhol, Robert Kennedy, W H Auden, Norman Mailer, Joseph Brodsky, Jasper Johns – pop up with dizzying regularity, providing vivid glimpses of a milieu you might expect to find in an upmarket version of Hello! magazine. A photospread of Susan Sontag inviting readers into her lovely penthouse flat in Chelsea, with views of the Hudson River, would hardly seem out of place.

Many people, Benjamin Moser observes in this meticulously researched book, would describe their first sighting of Sontag in ‘starstruck tones’. With her singular looks, effortless glamour and international renown, she inspired the kind of awe usually reserved for former presidents or pop stars. Not in everyone who met her, however: I have written before in these pages about the one occasion I encountered Sontag, at the Charleston literary festival in the summer of 2001.

At the time, I was chair of English PEN’s Writers in Prison Committee, and I had been asked to join other authors and actors in a series of readings. We were drinking tea companionably at the kitchen table when Sontag swept in, glowered at us and resisted all attempts to include her in the conversation. When a friend of mine persisted, Sontag issued a withering put-down, chiding her for her ignorance of Dostoevsky, and got on with giving the impression that we were beneath her notice. It was quite funny, in a horrible way. I remember kicking my friend under the table after the Dostoevsky comment and mouthing, ‘Didn’t you know that?’ I wondered what kind of desperate insecurity would make someone so gratuitously unpleasant to other writers. And my experience, it turns out, was far from unusual.

‘Many who encountered the actual woman were disappointed to discover a reality far short of the glorious myth,’ Moser writes in his introduction. ‘Disappointment with her, indeed, is a prominent theme in memoirs of Sontag, not to mention in her own private writings.’ It is a salutary warning to readers who have bought into the notion of the dazzling, supremely confident intellectual. Eight hundred exhausting pages later, Sontag emerges as one of those irredeemably unhappy people, endlessly lurching between narcissism and self-hatred, who leave a string of uncomprehending friends, relatives and lovers in their wake. She wasn’t even interested in personal hygiene or health, forgetting to wash, wearing the same clothes for days at a time, smoking heavily and gorging herself on food at other people’s expense. A friend once arrived late for dinner with her at Petrossian on 58th Street, only to find that Sontag had already gone home, leaving him with the bill for the ‘sumptuous, multi-course caviar dinner’ she had consumed.

So much for the gossip, but what about the writing? Moser’s book offers such a gripping account of a profoundly damaged human being, trapped in a cycle of repetition, that it would be easy to forget the fact that Sontag was a writer. It isn’t that he fails to discuss her essays and novels; on the contrary, he writes about her best-known work at perfectly respectable length. The problem is Sontag herself, who is quickly unveiled as a monster of selfishness. She was also, according to this account at least, perpetually in denial – both about her lesbianism, which she refused to acknowledge publicly, and about the disjunction between the ideas in her essays and the way she conducted her life.

As late as 2003, a year before her death, Sontag was still dissembling about her long relationship with the photographer Annie Leibovitz. ‘I am of course aware of these rumours,’ she told the New York Times. ‘No, the information is not correct.’ It was not just a lie but also a remarkable display of ingratitude, given that Leibovitz had supported Sontag financially for years, paying off the mortgage on her apartment in Chelsea, renting a second flat in Manhattan for her to work in and even giving her an apartment in Paris. All of this largesse came from a woman whose profession Sontag had subjected to savage criticism in a well-known series of essays published between 1973 and 1977 (later collected in On Photography), in which she argued that modern photography had turned people into voyeurs.

Another early essay, ‘Notes on “Camp”’, was written without any reference to its author’s tortured sexual orientation. Full of what now feels like gay stereotypes, it was originally titled ‘Notes on Homosexuality’. The decision to change it confirms how fraught Sontag found the subject. Her reticence about her own sexuality was understandable when the essay was first published in the Partisan Review in 1964, but hardly so almost four decades later, when attitudes to gay men and lesbians had changed dramatically. Similarly, in her 1978 work Illness as Metaphor, Sontag challenged judgemental ideas about cancer without acknowledging that she herself, having developed breast cancer and undergone a mastectomy three years earlier, was deeply imbued with those very ideas. ‘I’m responsible for my cancer,’ she had written bleakly in her journal.

It is hard to imagine a more comprehensive account of Sontag’s life than this one. Moser is by no means unsympathetic, arguing that Sontag was the typical child of an alcoholic parent, always trying to please her unstable mother and turning lovers into mother substitutes. Her father, Jack Rosenblatt, died when she was five and she eagerly adopted her stepfather’s surname, claiming that the name Rosenblatt was ‘ugly and foreign’ – and, according to a document quoted in the book, that it sounded too Jewish. When she married Philip Rieff, an academic at the University of Chicago, two weeks before her eighteenth birthday, she did not take his name. Moser’s suggestion that Sontag was the true author of Rieff’s book on Freud has attracted quite a bit of attention; if correct, it is fascinating to think that the first work of such an egotistical woman was published under someone else’s name.

What does all this information mean for Sontag’s reputation as a thinker and writer? She wrote about deeply personal subjects in highly judgemental terms, without acknowledging her investment in them, and the double standards she displayed in her public and private behaviour make the charge of hypocrisy impossible to avoid. Moser tries hard to be fair, concluding diplomatically that Sontag ‘both summed up and opposed her age’. But his book reveals the very worst traits of the writer-cum-celebrity, documenting a vast trail of emotional wreckage of the type more often found in showbiz magazines. If Susan Sontag was suspicious of interpretation, as she argued in a famous collection of essays, it’s not hard to see why.