Kevin Federline is fighting dirty for his kids - and Britney Spears's cash

If Britney Spears thought that Kevin Federline was going to walk out of their marriage quietly, she has another thing coming. Federline is believed to be preparing a tell-all book about his wife, in hopes that he can either make one last buck off of their relationship or win full custody of their children.

A source for Star magazine claims the book will cover Britney's drug use, bisexuality and her belief in time travel. Yes, time travel. The insider says, "Kevin is either hoping a publisher will pay him big money for the book or that Britney will cough up more cash than what's in their prenup [an estimated $360,000 in spousal support plus $7 million from the sale of their Malibu, Calif., home] to keep him from spilling the beans."

Kevin hopes that threatening Britney with the book will get her to hand him an extra $20 million. He's also going after full custody of their two sons, Sean Preston and Jayden James. Federline believes that the tales in his book will "prove that Britney is an unfit mother," says Star's source.

Also assisting Kevin in his case are his pals, who have been going to the media to declare his great parenting skills. One of those friends, DJ Irie, told Star that Federline is "going to fight hard. He wants his kids with him. There's no question about it. He's a great dad!" Irie also said that Kevin is trying to clean up his act in order to project a better image of himself to the world. He said, "[Kevin has] switched his look to 'GQ.' It's just a new phase in his life. He really is just trying to walk a straight line…It's not just something you're going to see today and tomorrow - it's going to be there for awhile."

DJ Irie also spoke about the activities that Federline likes to participate in with his sons. He said, "Their number one, favorite thing to do is watch TV together. He puts Sean on his lap, and he just sits back and watches TV. When I'm there, that's what he's always doing. They watch cartoons…When they go out, he likes to take them shopping. He even loves taking them to the supermarket…He spoils them. He gets them the most expensive stuff."