At Paperbackswap.com you can list your used books and if someone requests them you receive one credit for each book you ship to someone else. Each credit can be used to request a book that you want. You can also buy credits for roughly $2 a piece. Not a bad price to have a book brought right to your front door. There is no shipping charge so a book only cost a credit. You can get a credit for $2 or you can ship someone a book.
Here are some books at the site that I recommend:
Into the Wild (one of my favorite nonfiction books)
Blind Man's Bluff: The Untold Story of American Submarine Espionage
The Mosquito Coast