I just found a really fun site! It is called Paper Back Swap (but they do hardbacks and audio books too). If you look on the right and scroll down you will see a blue banner there that you can click to learn all about it yourself. Esentially you list 10 books that you are willing to part with, and you get two free credits and then every time someone “purchases” one of your books you receive another credit to “buy” a book. You have to ship your books out that are “for sale” obviously, and then when you buy books the person “selling” them ships them to you for free.

Have fun!