If you either have books you are looking at getting rid of or if you are looking for books, one source is Paperbackswap.com. Once you set up an account you can list post books you have to swap or start a wishlist of books you are looking for. You receive a credit for each of your books once the other party has received the book. You only pay for the shipping of the book, which is media mail so the books I have mailed have been between $2.50 and $3.00.
I have been using the website for the past couple of months. You may be able to find books cheaper at other sources such as thrift stores, but there may be more variety at the paperbackswap.com site. You can also start a wishlist of books you are looking for and then you will receive an email when someone posts the book. I have been using it for books that I am interested in keeping for reference or that I would want to keep longer than the 3 week checkout period of the library.
Even though the website is called paperbackswap, you can post and receive paperbacks, hardcover books and audio books. They also have new books available that you can purchase for 1 credit and an additional fee.