Awhile back, Inside The Net had the creator of BookMooch on their podcast.  I really enjoy using BookMooch.  I have an account on there and also at PaperBackSwap.   For the most part, both sites are wonderful and a good way to get rid of books I do not want.  I have scored some weird and very interesting books via BookMooch.  There are a few authors I always search for when I have a credit and I have been able to acquire some great books.