Kevin Federline is set to write a revealing book about his estranged wife Britney Spears.

The wannabe rapper, who split from Britney last month, is reportedly prepared to talk about the singer's alleged drug use, wild partying and her lesbian tendencies if he doesn't get what he wants from the divorce settlement.

A source told America's Star magazine: "Kevin is either hoping a publisher will pay him big money for the book, or that Britney will cough up more cash than what is in their prenuptial agreement to keep him from spilling the beans."

It has been reported that the prenuptial agreement entitles Kevin to $US360,000 in spousal support and $US7 million from the sale of their Malibu marital home.

Kevin has applied for sole custody of the couple's two children - 15-month-old Sean Preston and three-month-old Jayden James - and is reportedly seeking a settlement of at least $US20 million.

He is said to be threatening to "prove Britney is an unfit mother" by showing a video of the Toxic singer talking incoherently.

The source added: "Kevin says he filmed it just a week before Britney filed for divorce. He told me, 'I want the world to see that Britney is an unfit parent and that she's not in her right mind.'"