A few months ago I wrote a post about "Netflix for books" - services that send you books as a membership model. The Washington Post Sunday Edition has a collection of social networking sites where users can exchange books. This reminds me of my student days where book exchanging was more of a economic necessity.
The Washington Post article lists these sites :- BookCrossing.com , Bookins.com, BookMooch.com , novelaction.com and PaperBackSwap.com. You should read the article in the Washington Post Sunday Edition for more information.