You join for free at their website and enter the ISBN codes (found on the back of the book) of the books you are done with. If other members want them, they request them from you (your account is linked to your email) and you mail them off. Each book you send out earns you 1 credit, and you can request any book you want that is currently available. FOR FREE! :)
Now, you do have to pay shipping on all the books you mail out - but that's always been less than $3.00 for me. Pretty good deal, even for used books - especially because you can search for exactly what you want.
AND if they don't have what you want right now, just put it on your wish list and as soon as it's posted into the system they will notify you, so you can have it sent to you right away. Pretty cool.
Here's the link if you are interested. http://www.paperbackswap.com/home.php