...because cool stuff happens when you polish your books. Dust bunnies, mostly. And memories.
Anyway, check out the online book swapping site at BookMooch. Basically, you earn a point for every book you mail out, and you can spend points on books other people have posted. You pay for shipping costs, yes, but the site has an interesting selection of books. You can also set up a "moochbar" on your Amazon account to put in a request for books at BookMooch instead of paying for them through Amazon.