Henderson, who played the wholesome mother in that beloved family sitcom, was cheating on her real-life husband.
The actress also wrote she once got a minor sexually transmitted disease after a one-night stand with a former mayor of New York City.
"What a way to put the kibosh on a relationship," she wrote.
She also shoots down the myth she had an affair with Barry Williams, who played her TV son Greg Brady, but said he did have a crush on her.
The book, "Life is Not a Stage," is set for publication in September.
