On February 25, 2012, Benjamin Moser got an email with the subject heading: REALLY BIG QUESTION. He opened it, and it was indeed a really big question. Susan Sontag’s son, along with her agent and publisher, wanted to know if Moser was interested in writing the authorized biography of the legendary intellectual, who died of leukemia in 2004.

“It was really scary,” says Moser, author of Why This World: A Biography of Clarice Lispector, “because I knew what a biography was. I knew how difficult and expensive and long-term they are. It is sort of like an arranged marriage. You’re not sure who you’re walking down the aisle with, and then you’re there with that person for the rest of your life. It doesn’t end when the book is published. That is just the start of the relationship. So you have to ask: Do I want this person in my life? I couldn’t resist her, obviously.”

It was Moser’s first arranged (biographical) marriage, with the Brazilian writer Clarice Lispector, that had caught the attention of the guardians of Sontag’s legacy. They decided that he was the right man to spin biographical gold out of the life of another brilliant, cosmopolitan woman who’d made her mark on the 20th century.

They were right. The biography, Sontag (Ecco, Sept. 17), is exceptional. It’s a fast-moving, intellectually rich, deliciously intimate and psychologically savvy profile of a woman who didn’t just write brilliantly about a million different things—including AIDS, photography, film, the camp aesthetic, cancer, Bosnia, and Sigmund Freud—but lived and loved and loathed with a persistent intensity over many decades.

“She was at the Grand Central Station of every important and interesting person and event in the 20th century,” says Moser. “The book starts off with the Armenian genocide and then ends with the genocide in Yugoslavia. You couldn’t make Susan Sontag’s life up. Oh, look, the Berlin Wall is falling—right as Susan is walking out of a movie theater in Berlin. I think if you put Susan in a novel, and a lot of people did, it wouldn’t be believable.”

In taking on such a monumental life, it helped that so many people who were important to Sontag talked to Moser. This included her son, David Rieff, who had a close and tortured relationship to his mother; the famed photographer Annie Leibowitz, who was Sontag’s partner for most of the last 15 years of her life; a host of former lovers of both sexes; and a Who’s Who of eminent American and European intellectuals and artists.

“She had that characteristic of getting into people’s heads,” says Moser. “You can’t overstate how important Sontag was to so many people. She was the ex-boyfriend you’re still talking about 10 years later.”

Moser believes it was this extraordinary charisma, as much as his official designation as her biographer, that elicited such candor from so many people.

“People would act out their feelings toward Susan on me,” says Moser. “If they thought I loved Susan and they didn’t, they would act that out with me, as would the people who loved Susan and assumed I didn’t. The Freudians call it transference. The therapist (or in this case the biographer) becomes your parent. Not everyone did this, of course, but I saw it happen several times. But even when it doesn't, you realize how much people want to tell their stories, want to be listened to. And that includes world-famous people.”

By the end of the book it is evident that Moser has come to love his subject. He does so with clear eyes, however.

“I got turned off at some points,” he says. “It happened to everyone who knew her. But I really love and admire Susan, and I want that to come across. I’m not naive. I know how she could be to people. It’s OK to be difficult. Too often we want someone to either be a monster or some kind of sainted ceramic figurine, wreathed with incense. She was both heroic and impossible—for other people but especially for herself.” 

Daniel Oppenheimer is the author of Exit Right: The People Who Left the Left and Reshaped the American Century.