As a snooty writer, I'm not sure how I feel personally about deciding that you want to write a book before you actually have an idea for one. Probably because deep down, after my years of laboring over my own half-baked ideas, I'd be jealous of somebody who went that approach and then published. But if you do work that way and get published, the more power to you and your ambition. Anyway, Nick Daws at WorldWide Freelance Writer has some ideas for your idea for a book, if you don't have any already.