I'm sure most of you know what a book worm I've become. Some kind of computer book junkie. Well, I buy quite a bit from Amazon.com and I typically find what I'm after in their Used Books section. I'm also a big fan of Halfprice Books. I also check out are the sales tables at book stores and stuff. For some good deals, not just on books, check out Overstock.com. Anyways, without listing too many more sites, I found most of them linked from ISBN.nu. What you do, is find the book you're after, give it to isbn.nu and it will search all of the best online book retailers and give you the breakdown of the pricing. (kinda a searchable-salescircular.com for books)
FYI: PHP Cookbook is freaking awesome!