The question of what was real and what unreal — philosophically or on the ground — lay behind almost everything Sontag ever wrote about, whether it was writers or philosophers, politics or aesthetics, illness or photography. During periods of political activism that landed her in actual war zones — Vietnam or Sarajevo — she did feel right up against herself and could make common cause with all around her who felt the same. However, nothing could induce her — neither war nor new love nor her 30 years’ battle with cancer — to frame a piece through the use of her own immediate experience. Although it was ever the abstract rather than the concrete that sparked her writing imagination — the political theorist Herbert Marcuse once said disparagingly, “She can make a theory out of a potato peel” — it was this very passion for experience in the abstract that gave her the subjects out of which she made glorious a form that had long been neglected.

But the generative power of the intellectual essay deserted Sontag in midlife and then, desperate to rejuvenate her work, she took (in the opinion of this reviewer) a decidedly wrong turn. In her youth she had written two abstract novels, and then let novel-writing go. Now, in the 1990s, she determined on returning to fiction, and produced two historical novels — “The Volcano Lover” and “In America.” The writing in each of these books is elegant and original, at times even glittering, but at the last it fails to put felt life on the page. The one thing she longed all the years to do — make art — lay beyond her powers.

Benjamin Moser’s biography is a skilled, lively, prodigiously researched book that, in the main, neither whitewashes nor rebukes its subject: It works hard to make the reader see Sontag as the severely complex person she was. But Moser doesn’t love her, and this absence of emotional connection poses a serious problem for his book. A strong, vibrant, even mysterious flow of sympathy must exist between the writer and the subject — however unlovable that subject might be — in order that a remarkable biography be written. And this, I’m afraid, “Sontag” is not.

During the course of its 800-plus pages, Moser (who has also written a biography of Clarice Lispector) describes in detail every love affair of Sontag’s, every intellectual position she took, every famous acquaintance she made, every prize and award she received. He praises her for what is praiseworthy and, more or less, holds her accountable for what is not. But it strikes me that because he doesn’t trust his own feelings, he often fails to probe as far as we’d like him to. There are times when this timidity twists his paragraphs into distinctly odd shapes. For instance, in her famous essay on pornography Sontag weaves theoretical constructions for so long that they often seem to fall far from anything that resembles verifiable experience. Here’s how Moser deals with this difficulty:

In a single paragraph, he tells us that Sontag writes, “What pornography is really about, ultimately, isn’t sex but death.” Moser then writes, “This seems questionable: Pornography, at least for most people, really is about sex.” But come to think of it, he says, “etymology relates pornography to the broader themes in Sontag’s work. In Greek, pornografia means a ‘depiction of prostitutes.’ And it is not prostitutes but the depiction of them that relates pornography to death. Depictions — images — show lives bound toward their undoing.” Huh?

If I have any other complaint about the book, it is that it is somewhat psychologically reductive. Repeatedly, it returns to the negative influence of the alcoholic mother — as though growing up the child of an alcoholic could explain a Susan Sontag — and repeatedly, it dwells on the fame that assaulted rather than gratified her, certainly never put her demons to rest. Somehow, neither of these explorations allows Moser to dive deep. On the other hand, he writes vividly of a woman of parts determined to leave a mark on her time; and makes us feel viscerally how large those parts were — the arrogance, the anxiety, the reach! No mean achievement.