www.paperbackswap.comwww.bookmooch.comSomeone somewhere has the book you want, or wants your book. If you're looking to get rid of books or pick some up used, I recommend these two sites.
There might be a little bit of a wait, but if you 'Need That Book Now,' there's always the local store. Game books are kind of thin, but general non/fiction abounds.
I've tried a bunch of other sites (5?) but these two have given consistently good experiences.