This site is fairly new and it invovles you listing you used paper back book that you wan to get rid of. Then a person comes and request that book. You print out a premade wrapper withy the persons address and then send it to them via media mail about $$1.50 a book and then recive a credit to request books that you want. This site is great it also has many different things on it.
There is Box-Of-Books (BOB) where you pick a number of books from a persons list and they pick the same amount from you list and then you swap no credit's needed. The site also keeps a list of books read for you. You can also keep a reminder list of books you want and a wish list for Books not posted yet.
When you search their data base of over 800,000+ books you can also search unposted books and then add them to your wish list. Once some one posts a book on your wish list you get the opportunity to request it.
This site is great for the people who love to read but hate buying new books and you can also get rid of books so you don't have a huge stash sitting around.
http://www.paperbackswap.com/
Just thought I would mention a couple of other websites that PaperbackSwap.com came out with that are just as great!
1. SwapaDVD.com: Exchange DVDs & Movies for free.
2. SwapaCD.com: Exchange Music CDs for free.
Enjoy!