Susy’s parents divorced when she was in the second grade, and her mentally unstable mother loomed over her childhood like a frightening mythological creature — part Fury, part predator, part companion and guide. Her mother, whom she calls Daphne — Ms. Sonnenberg says she’s changed all names except her own in the book “to emphasize that this story could only be mine” — possessed a magical charm and a contagious, manic enthusiasm, especially in her “Big Everything” frenzies, her “Let’s-Take-Over-Central Park” moods.

And Susy cherished her mother’s soul-searching talks. “No one else made me feel really interesting, different, magical,” she writes, no one else wanted to know all her “thoughts & feelings.” But she also recalls that Daphne was capable of turning from conspiratorial affection to accusatory rage in a flash, hurling insults, punches and undermining invective at her daughter, while lying, seemingly reflexively, about everything from the men she dated to her imminent death from a fatal illness.

The worst, however, were Daphne’s falling-down, passing-out drug binges and her relentless obsession with sex, which she shockingly shared with her young daughter. According to Ms. Sonnenberg, her mother told her about orgasms when she was 8, and years later would quiz her voyeuristically about the sexual details of her relationships with boyfriends. Daphne flaunted her string of lovers — including a New York Giants quarterback, a BBC executive, a Broadway actor, a Sotheby’s executive, a movie theater owner and a famous lyricist — in front of Susy and her younger sister. And when Susy started dating, she says, her mother would routinely try to seduce her boyfriends.

Daphne, who suffered from chronic back pain, was equally promiscuous with drugs, Ms. Sonnenberg recalls, downing Demerol, Percodan, Valium and sleeping pills with wild abandon, even as she cultivated a serious cocaine habit. As a young girl, the author says, she was forced to help her mother inject herself with Demerol, watched her mother pass out time and again, and became accustomed to visiting her in the hospital.

Throughout these episodes Susy behaved as the resident grown-up, alert to her mother’s disasters and intent on trying to shield her younger sister from Daphne’s worst catastrophes. She got herself and her sister ready for school every morning, scheduled their dental checkups, thawed peas, scrambled eggs, turned off the television when they had watched enough. She resisted taking peyote with the grown-ups — “Susy’s our straight one!” her mother would say with a laugh — and resisted her mother’s efforts to push her early into sexual relationships.

But Daphne, with her penchant for lying and sexual manipulation, would also become a kind of role model for her older daughter, a siren call to the dark side. After an intense affair with her high school English teacher and a series of serious college-era boyfriends, Susy embarked on an extended interlude of promiscuity, going “to bed with everybody,” using sex “to court oblivion” and becoming “a virtuoso of the lie.”

It was meeting her future husband, Christopher, a man as earnest as she was ironic, as plain-spoken and straightforward as she was sophisticated, that put a punctuation point to this phase in her life and helped her begin a new chapter in which she could start to exorcise her mother’s spirit. Susy would move to Montana with Christopher, marry him, have two sons and begin a no-frills life of blessedly ordinary pleasures, frustrations and rewards.

She would also distance herself from her mother, out of self-preservation. When Daphne was injured in a car accident, and Susy was summoned to Barbados, she declined to go.

Throughout this book Ms. Sonnenberg refrains from analyzing her mother’s behavior — there is no speculation in these pages about mental illness or chemical imbalances or emotional damage — and she refrains as well from speculating about the psychological fallout of growing up with such a parent. The reader suspects that this aspect of “Her Last Death” was by design, perhaps from an inability (or unwillingness) to deconstruct her mother’s behavior, perhaps from a desire to avoid committing the sort of dime-store psychoanalysis so popular on daytime talk shows.

This approach makes, at times, for a curiously opaque narrative, but it also intensifies the immediacy of Ms. Sonnenberg’s story, plunging readers into a sort of perpetual present tense in which we are made to experience, almost firsthand, the inexplicable and perverse behavior of an impossible woman from the point of view of her aghast, bedazzled — and immensely gifted — daughter.