Well - I was recently introduced to a used bookstore that buys back your books for cash or credit at the store.
Husband and I took a great big box FULL of books to the place... and, in return for our old books, were able to pick up several CDs and new (to us, at least) books. Was pretty sweet.
Maybe check out Google Local for used book shops in the area?
Additionally, you can donate them to Goodwill, the Salvation Army, or the local hospital and - possibly (but I could be wrong) claim it as a charitable donation on your taxes.