All her life, Susan Sontag, a voracious moviegoer, insisted on sitting in the same seat in theaters: third row, center.

I can’t recommend it, not if you value your neck. But it’s where she sat, and the very place she occupied in the culture for close to a half century — up front, in the midst of the action — establishing the tone and terms for debates on taste, language, global literature, ethics and photography, involving herself in matters of military intervention and genocide. Not for her the stately remove of the American intellectual, the retiring panel-dweller — never mind the risk to her neck; Sontag was in Sarajevo during the Bosnian War and Berlin as the wall fell. She was the striking subject of photographs by Richard Avedon, Diane Arbus and Robert Mapplethorpe — a Zelig-like figure who even made a cameo in “Zelig.”

Sontag died in 2004, of complications of leukemia. Since then, another essay collection has been published, along with the first two volumes of her diaries, chronicles of her “sore heart + unused body” — the one place where she wrote frankly about her homosexuality and grieved for her unhappy relationships. Monographs, memoirs and remembrances have followed, including her son David Rieff’s account of her final illness, “Swimming in a Sea of Death”; Phillip Lopate’s affectionately rivalrous study of her work, “Notes on Sontag”; the scholar Terry Castle’s tart essay on their friendship; and the novelist Sigrid Nunez’s “Sempre Susan,” an account of sharing an apartment with Sontag when Nunez and Rieff dated in the ’70s. To add to the shelf, there is now the authorized biography, “Sontag: Her Life and Work,” by Benjamin Moser, a book as handsome, provocative and troubled as its subject.

One note recurs through many Sontag stories. It is puzzlement — ranging from the amused to the appalled — at the gulf between her public preoccupation with ethical action and some truly filthy private behavior. She was prone to fits of grandiosity and torrential abuse, particularly to her longtime partner, the photographer Annie Leibovitz. She was said to oscillate, as a mother, between neglect and invasive closeness. There was “Good Susan and Bad Susan,” Salman Rushdie wrote in his memoir, “Joseph Anton.” “Bad Susan could be a bullying monster.”