You can never have enough sources for gently used books, so I was glad to learn that I missed some good bets when I wrote about used books a few weeks ago.
And I heard from several folks about paperbackswap.com -- a free book-swapping site that's a great way to get and get rid of books.
Just go to the site and list the books you'd be willing to send to other readers with their ISBN number.
If requested by another reader, the only cost to you is the postage, and there is no cost to the person receiving the book.
You get a credit to order a book for every book you send.