I just found the coolest website, paperbackswap.com. I read about it on Real Simple. You simply list the paperbacks you want to swap out and the for the first 9 books you post you get 3 free swaps. Then you can pick from thousands of paperbacks to get for FREE! Everytime you swap a book out with someone, you get another credit for a trade.
Of course, you send the books media mail which takes about 20 days… but, you get FREE books! Check it out.