The 49-year-old former star of TV's "One Day at a Time" has long battled an addiction to drugs. Her memoir, "High on Arrival," goes on sale Wednesday.
John Phillips died in 2001 at age 65.
"On the eve of my wedding (to musician Jeff Sessler,) my father showed up, determined to stop it. ... I had tons of pills, and Dad had tons of everything too. Eventually I passed out on Dad's bed," People.com quoted her as writing in the book.
"My father was not a man with boundaries. He was full of love, and he was sick with drugs. I woke up that night from a blackout to find myself having sex with my own father. Had this happened before? I didn't know. All I can say is it was the first time I was aware of it. For a moment, I was in my body, in that horrible truth, and then I slid back into a blackout."
She said the sexual relationship became consensual.
"I was a fragment of a person, and my secret isolated me," she wrote. "One night, Dad said: 'We could just run away to a country where no one would look down on us. There are countries where this is an accepted practice. Maybe Fiji.' He was completely delusional. 'No,' I thought, 'we're going to hell for this.'"
The People.com report did not say when or how the relationship ended.