I've encountered but a single book specifically on Fantasy and Science Fiction writing: "How to Write Science Fiction & Fantasy" by Orson Scott Card, the guy who wrote Ender's Game among others. The book's been around for a while (first published in 1990, I believe) so some of the later sections about the business of writing may be out of date. Earlier parts of the book, though, focus on areas such as consistent world building, the rules of magic, space and time travel, and a few other nuggets specific to SFF.
It's a fairly thin book. I certainly found it interesting, but if you're already familiar with the SF and Fantasy genres it may be of limited use. It works best, I would say, for someone who has not read widely within the genres. At least, that's how I remember it: I read it quite some time ago.
If you've read a lot of Fantasy and Science Fiction then the chances are you already understand a lot of those areas specific to SFF, in which case you'd probably find one of the general fiction writing novels mentioned by others in the thread more useful. Definitely worth a read though if you've got the budget.