I have found Paperbackswap to be a great resource for me. I am able to trade away my older books that I don’t plan on reading again for credits to get other books to read. In the last two years or so I have mailed out 31 books and received 23 back in return. The other 8 credits just sit in my account until I’m ready to order a new book. The only down side is that some of the popular books might have hundreds of people in the queue so it can take a long time to get a particular book (if ever).
If it sounds interesting use the linked image below to sign up so they know Doppelbock sent you.