You should: http://www.swaptree.com
This is a useful site if your parents-in-law are super-fast readers who blow through mass-market fiction and leave stacks of nearly-new paperbacks in your guest room.
I only trade books that I don’t really like, so that limits the selection of books I can receive. If I were to trade all the once-read favorites on my bookshelf, I’d expand my trading choices quite a bit.
Went in the mail Saturday: Marley and Me
Receiving in exchange: Midnight in the Garden of Good and Evil