What would Susan Sontag (1933-2004) have made of 2019? The champion of artistic will would have confronted neurobiology’s assertion that free will doesn’t exist. Her commitment to high culture would likely have clashed with Twitter clickbait. Her famous “Notes on ‘Camp’ ” now feels quaint and grandmotherly compared with the transgendered stylings of Pose.

And what would 2019 have made of Susan Sontag? We have an answer, and it’s glorious. Benjamin Moser’s authorized biography, “Sontag: Her Life and Work,” is an epiphany of research and storytelling, the definitive life of a writer both more and less than the myth she fastidiously crafted.

In Moser’s telling, Sontag was a bundle of contradictions: an advocate for LGBTQ culture who cloaked her own homosexuality; a Eurocentric snob who adored Japanese science-fiction films; an activist whose words and deeds often ensnared her in controversy; a glamorous public intellectual whose private life was a hot mess.

Raised in Arizona and suburban Los Angeles, Susan Rosenblatt lost her father at the age of 5; as a teenager she took her stepfather’s last name. Her mother, a beautiful alcoholic, dominated her daughter. While a precocious student at the University of Chicago, Susan married a professor, Philip Rieff — among Moser’s many juicy revelations is Sontag’s authorship of Rieff’s book on Freud — and gave birth at 19 to her only child, David, with whom she would share a complicated, adventurous history.

Sontag strides across “Sontag” like a colossus. After her divorce, “tall, olive-skinned … with the mind of a European philosopher and the looks of a musketeer,” she pursued romances with women as she published masterworks — “Against Interpretation,” “On Photography,” “Illness as Metaphor” — that cemented her reputation as America’s leading critic. Diagnosed with cancer at 42, she fended off the disease for three decades. She popped speed to stave away sleep, binge-writing for hours at a time. She hobnobbed with artists and celebrities alike. Her pen was penetrating even as she acted out selfishly with intimates, including her later-life companion, photographer Annie Leibovitz.

All of which is to say: Sontag was a brilliance like no other, sexy and scintillating and smug, equal parts Jean-Paul Sartre and Marlene Dietrich. But Moser’s no hagiographer; he details Sontag’s middling fiction, her petty grievances, a childlike inability to take care of herself. (She would forget to bathe for days.) He’s harshest on her cluelessness during the AIDS epidemic, how she barricaded herself in the closet, but also plumbs one of Sontag’s finest moments, when she directed a candlelit “Waiting for Godot” in war-gutted Bosnia: “If praise and prosperity brought out the worst in her, oppression and destitution brought out the best. If she could be haughty in New York, she was kind in Sarajevo.”

A searching meditation on the divided self, a warts-and-all appraisal of Sontag’s behavior, scrupulous readings of her texts: all speak to Moser’s luminous achievement. In a moving coda he summarizes a career that dared to go against the grain of American conventionality: “She showed how to remain anchored in the achievements of the past while embracing her own century. She demonstrated endless admiration for art and beauty — and endless contempt for intellectual and spiritual vulgarity. She impressed generations of women as a thinker unafraid of men, and unaware she ought to be.”

 Hamilton Cain is the author of “This Boy’s Faith: Notes From a Southern Baptist Upbringing” and a member of the National Book Critics Circle. He lives in Brooklyn.

Sontag: Her Life and Work
By: Benjamin Moser.
Publisher: Ecco, 816 pages, $39.99.