Although I don't have any means for comparison, I have to say I thought the CD that ETS sends you when you register, along with the resources on the ETS website, make for excellent preparation. I found the math review and the pool of writing questions to be very helpful. Another advantage of the ETS CD over other books is that you can take practice exams under nearly-exam-like conditions and get a feel for how the process works. Also, it's free!
If you're really more of a prep-book sort of person, I know this won't be of much use to you. I figured that in case you've just signed up, or are just thinking of signing up to take the test, and have some time to prepare, you might want to wait to get the free materials (or access them online now) before you sink additional money into prep books.
Good luck! 😎