Paperback Swap

There is a great place called Paperback Book Swap or PBS for short. What it is, is exactly what the name suggests... a place to swap books. How it works is that you get a credit every time a book successfully reaches it's destination. You swap the credits for books. All hardcover and paperback book covers are all one credit each. All audiobooks are 2 credits each. You can get deals by going into the Bargain forum and can often get 2 or 3 books for one credit. You can also try to send many books to one address via media mail so it makes getting credits cheaper. 

I personally have been a member since last year. I've loved it. It's so much cheaper than buying the books and you can keep books as long as you wish. Once in your hands, it is yours and you do not have to return the book like you do at the library. Now, the books do have to be in "good" condition which is defined more specifically on the site, but generally means that the cover is intact, there are no stains on the pages and no water damage. 

It's a great place to try out and you get 2 free credits once you list 10 books. Happy swapping!