The book's title: Deer in the Headlights: My Life in Sarah Palin's Crosshairs.
"I want to tell the truth about my close relationship with the Palins, my sense of Sarah and my perplexing fall from grace -- how I feel and what I've learned," Johnston said in a statement released by Touchstone Publishing, which is an imprint of Simon & Schuster.
The book is scheduled for publication in the fall.
In the meantime, Us Magazine reports Johnston — a high school dropout who's posed for Playgirl and plans to run for mayor of Wasilla, Alaska — is planning to celebrate his 21st birthday in Las Vegas on May 6.
As to why he's ultimately doing the book, Johnston said in his statement: "I'm doing this for me, for my boy Tripp and for the country."