Do you have books that you don't want anymore? Try BookMooch. Enter the books you don't want on the site and earn points when you send the books to people who request them. Then, use the points to request books that you do want. I just signed up yesterday, and I already earned points for two books to send to other people. I used those points to request a couple books that I want. All it costs is the postage to mail books out (which isn't usually that expensive at the Media Mail rates).
I added a list of some of the books I'm giving away. Look on the right side of the main page of this blog.