Jessica and Nick were married from 2002 until 2006, and in her new memoir "Open Book", she insists her ex must have known the end was coming.
"We had been together almost seven years when I told Nick, 'I think I want a divorce,' " Jessica, who is now happily married to former footballer Eric Johnson (II), writes. "I later heard that he told the press he was blindsided. I don't know how. At that point we were not even speaking to each other."
"Maybe he was just shocked that I stood up for myself. I don't think he ever thought I would take the leap."
In the new book, Jessica also opens up about her romance with John Mayer and a sexual encounter with a family friend's daughter that scarred her as a child.
Meanwhile, Simpson is planning to relaunch her pop career in 2020, a decade after the release of her last album.
The 39-year-old is planning to drop six new songs to mark the release of "Open Book" on 4 February. The tracks will be included on the memoir's audio book, which Simpson narrated herself.