On April 27, 1984, 18-year-old Billy Gilley, a shaggy-haired, pot-smoking high-school dropout, entered his parents’ bedroom with an aluminum baseball bat and bludgeoned them to death in their sleep. When his 11-year-old sister, Becky, caught him in the act, he beat her, too (she died at the hospital two days later). Only his 16-year-old sister, Jody, was spared. “We’re free,” he said, summoning her from her bedroom shortly after the killings. “I’m not crazy. Do you think I’m crazy?”

Kathryn Harrison revisits this Medford, Oregon, parricide in While They Slept, her lucid, psychologically probing and disturbing new book. Relying on court documents and interviews with Billy and Jody, who later testified on behalf of the prosecution, Harrison weaves a narrative in which the facts are sometimes hazy but consistently point to an emotional truth: The Gilley family dynamic was marred by parental abuse, ridicule and Billy’s apparent love for Jody. She traces a pattern of violence and dysfunction that extends through several generations, the murders being its apex.

A novelist and nonfiction writer most famous for her memoir The Kiss, in which she recounts a painful four-year sexual relationship with her once-estranged father, Harrison uses her own psychic trauma as a frame of reference, reflecting on Jody and Billy’s lives “before” and “after” the cataclysmic event. Jody, who retreated from her parents’ tyranny into a world of Harlequin romance novels, eventually earned a degree from Georgetown and a job as a communications strategist in Washington. It’s the culmination of “a blood-soaked fairy tale,” the author writes. Perhaps, but While They Slept is no children’s tale. Rather, it’s a morally nuanced story, and a culmination of Harrison’s favored themes: sex, family and power.