I read a lot of books and decided to check it out. It is a free exchange of books and audio books. You have to list at least 10 books to start out and they will give you 2 credits. To earn more credits you just mail (via media mail) the books that you have on your list to people that want them. In return you receive credits to pick out books from others on the site. So, I signed up and tried it out. I received 2 free credits and ordered two books that evening and about a week the books arrived. They were both mailed within a few days of my request and since it is media mail it can take a bit longer for the books to arrive. So far I have mailed out 4 books and one of them arrived to its new owner yesterday. Once they received the book, they gave me credit and I was able to order another book.
It is an easy site to use and it costs under $3 to ship via media mail. So for approx $3 you get a new to you book. Not a bad deal at all.
They also offer a vacation hold - so if you will be gone or don't want any request for a period of time you can place your account on hold.
The website is http://www.paperbackswap.com/index.php