"My last book, How to be Famous, didn't make the New York Times best-seller list because it was in the self-help section," Pratt tells the magazine. "This one is going to be totally different. I'm not holding anything back."
He did not name a publisher.
Montag, who filed for a divorce last month after one year of marriage, has since expressed her anger about the planned tell-all.
"This is exactly why I left him," she tells TMZ. "Right now, I'm looking into my legal options."
Calls and emails to Montag's lawyer and Pratt's rep were not immediately returned.