Johnston fathered a child with Bristol Palin, the daughter of the former Alaska governor, when they were teenagers. The pregnancy was announced days after Palin was selected as the 2008 GOP vice presidential candidate. The couple broke up after the birth of their son, Tripp, and reconciled briefly.
He's had a contentious relationship with the Palins.
Johnston says the book, first reported by People.com Monday, will "tell the truth" about that relationship, including his "sense of Sarah and my perplexing fall from grace." He says he's doing it "for me, for my boy Tripp and the country."