I haven't titled a book yet, but for my stories, I nearly always start with thesaurus.com and work through various words that have to do with the story in some way. In the course of following trails through the thesaurus, something usually pops out that makes a good punny title, or at least a witty title, or sometimes just a solid double- or triple-meaning title. That's hardly ever failed me in the short story realm, so I hope it works for my book when I need it.
If the thesaurus fails, sometimes Google does the trick and brings up some odd connection that leads somewhere.
Sometimes I have a title pop into my head to write the story around, but that's rare.