S wollen into the definitive category through the use of oversize type, this is a big, heavy book. But whenever my eyes got tired, I revived them with another gander at its endpapers. Reproduced there is a contact sheet of 25 photographs of Susan Sontag. She is lying down, her head on her hands, smiling at Peter Hujar’s camera here, looking dreamy and distant there, coquettish in this one, come-hither in that. Sontag once granted Albert Camus the honorific of “ideal husband of contemporary letters”. Given what Benjamin Moser tells us about her slatternly habits (she hated baths and showers, and was once hosed down with disinfectant by a lover), it would be a stretch to label Sontag the ideal wife.

Still and all. “You’re the only person in the world who can call me ‘baby’ and get away with it,” Sontag once told her publisher, Roger Straus. Well, she isn’t around any more, so I’ll say it too: Susan Sontag was a babe — a body as well as a mind. Sontag was fond of Nietzsche’s line about how “experienc[ing] a thing as beautiful means: to experience it necessarily wrongly”. Certainly we get Sontag wrong if we take her only at face value. A babe she might have been but she was also a novelist, a critic, a filmmaker, a theatre director, a polemicist, a theorist, and all-round smart-cookie. So smart that by the eleventh grade she was reading Kant (or was until a teacher snatched it from her and offered her a copy of Reader’s Digest instead).

Around the same time she noted in her journal that she “believe[d] Schopenhauer to be wrong”. Soon she was knocking on Thomas Mann’s door (he lived in Pacific Palisades, a few miles south of the Sontag residence in San Fernando) to interrogate him about his recently-published Doctor Faustus. “Sue,” her stepfather implored, “if you read so much you’ll never find a husband.” How wrong he was. In college by the age of 15 (first at Berkeley, then at Chicago), Sontag was married a few days before her eighteenth birthday to Philip Rieff, a sociology lecturer. Not only was she still reading, she also, says Moser, wrote a book for her new husband.

He could be right, though the claim isn’t as new as he’d have you believe. The authorship of Freud: The Mind of the Moralist has been in question for years. Sontag’s son, David Rieff, has long suggested that his mother was pretty much the book’s onlie begetter. And back in the early noughties, during a Radio 3 profile, Sontag claimed that since her husband had been suffering from writer’s block she’d written the book for him.

Well she would say that, wouldn’t she? But while Sontag certainly helped with the book, there’s no proof she wrote it. Moser’s argument is that Freud is too good to have been written by a writer as dull as Philip Rieff’s later works prove him to have been. Fair enough, though we could turn that on its head and say that since Sontag never wrote a book-length work of non-fiction it’s unlikely that Freud was solely her work. Moreover, whatever we make of the book, it is indubitably at its best as a work of history — and as David Rieff once told his mother, “You know nothing about history.”

I labour this point because for all Sontag’s weight and girth, there is little that is new here. Moser needs his claim about the authorship of Freud to be true in order to have something on which to peg an 800-page biography of a woman who was basically a journalist. To be sure, Sontag wrote some justly influential pieces but they are rather fewer than this book’s breezeblock dimensions might suggest. Despite her lengthy dominance of the culture scene, all Sontag’s essential work was written between the early Sixties and the late Seventies. (It’s collected in three books: Against Interpretation, Styles of Radical Will and Under the Sign of Saturn.)

Subsequently she turned more and more to the writing of novels, though it would be a charitable critic who called this a good career move. Like Orwell and James Baldwin, whom she once described as “better writers in their essays than . . . in their fiction”, Sontag had too analytic an intelligence to thrive within fiction’s less systematic precincts.

Her novels are as desiccated as the god-awful Godardian films she made in the late Sixties. “I thought I was a teacher,” Sontag said towards the end of her life. “And then I discovered that I liked to tell stories and make people cry.” Anyone who has sat through Duet for Cannibals or waded through In America will tell you that there are two ways of reading that sentence.

She shared the New Left’s infantile dream that “psychic and political redemption are one and the same”

Shortly after the publication of In America, Sontag was accused of plagiarism. Ellen Lee, whom Moser calls an “amateur historian”, pointed to a dozen or so passages in the novel that were lifted verbatim from other books. Sontag didn’t demur. Instead, she came up with some po-mo blather about intertextuality and how “all of literature is a series of references and allusions”.

We could argue about that forever (for what it’s worth, Moser believes Lee had Sontag bang to rights). But Sontag’s defence is a nice reminder that one of her biggest achievements was the popularisation of some of the twentieth century’s more challenging theorists.

Intertextuality, for instance, was an idea made famous by Roland Barthes, about whom Sontag wrote persuasively on several occasions. The title essay of Against Interpretation is both a refutation of Freud and Marx and an introduction to the ideas of the Russian formalists. And Sontag was one of the first writers to big up Walter Benjamin, a great critic and the key influence on her one unimpeachable book, On Photography.

But she wasn’t always on the money with the continental philosophers. Anyone who pronounces themselves “very interested in Baudrillard’s perspective” needs to get some perspective of their own. And anyone who can applaud Sartre for “lucidity” needs to see an optician.

Not that Sontag was blind to the world. Her essay on Vietnam, “Trip to Hanoi”, is one of the Sixties’ ur-texts. And she showed great bravery during the Bosnian war, when she staged Waiting for Godot in Sarajevo. But as with philosophy, her political antennae were erratic. She shared the New Left’s infantile dream that “psychic redemption and political redemption are one and the same”. And she never got past the adolescent urge to see private problems as world events. Undergoing chemotheraphy for the first of her three bouts with cancer she told friends she felt “like the Vietnam war”, a metaphor whose tastelessness is only compounded when you recall her declaration during that war that “the white race is the cancer of human history”.

Moser isn’t terribly interested in Sontag’s politics. What intrigues him is her sexuality. Although she had a number of male lovers — among them the painter Jasper Johns and the Labour Party’s Bernard Donoghue — Sontag was gay from the get-go. Alas for her, she was unhappy about it. At 16 she was diarising about “the incipient guilt I have always felt about my lesbianism”.

Since that was back in the late 1940s when, Moser reminds us, “homosexuals were commonly seen as perverts and criminals”, she had good reason to feel bad. But couldn’t she, he wonders, have come out in later life, when everybody knew that she and the Vanity Fair photographer Annie Leibovitz were an item? Apparently not. Instead, she sought to cover up the affair by dissing Leibovitz in public for being dumb and not having read Balzac.

Sontag was as well-read as anyone but she wasn’t a great critic of the non-literary arts. As more than one person tells Moser, she had no eye and no ear. She often went without sleep while writing (keeping herself going on “Marlboros; bottles of Dexedrine [and] rivers of coffee”), and said she got through life by “pretend[ing] my body isn’t there”.

But the body must be there if a critic is to react fully to a work of art. You can’t begin to get a handle on a Holbein miniature or an ab-ex whopper without measuring their presence against your own. Not for nothing were Allan Kaprow and Jim Dine the only artists Sontag ever wrote about convincingly. The idea was everything with their “happenings”: you didn’t have to be present to get the gist.

By the end of her life, Sontag could ignore her body no longer. Cancer finally killed her in December 2004, a couple of weeks before her seventy-second birthday. Not that she went without a fight. She had a bone-marrow transplant that “entailed grotesque suffering”. Her skin turned black and “her face swelled beyond recognition”. Leibovitz, who paid for the operation, took pictures.

I haven’t seen them but the film historian David Thomson has and says, “Really, it’s a matter of when you find yourself throwing up.” That’s less a criticism than a cri de coeur, a reminder — as so little of Susan Sontag’s work is — that we respond to art through more than the mind.