My friend and Auburn, NY librarian Anne Mlod recommended Paperback Swap to me. She loves it. The idea is simple: you post some of your books on the site, then search through the catalog for some that you want. Your only cost is basically the postage for mailing it out. Every book owner is responsible for mailing their books to whomever requests them, and the site even creates a wrapper you can print, complete with the address and postage pre-calculated.

http://www.paperbackswap.com/