A site I use quite a bit is www.bookmooch.com. This site is a great way to trade books that you multiple copies of in your library. The short story about this site is that you can search through thousands upon thousands of books and trade your books for something that might interest your patrons. After you list the books that you will part with you will be able to get books from someone on the site. no cost to you, just send them an email and they will send it to you. In return if someone wants your book you will send it to them, you pay for postage on the books you send out.