Today, the intrepid soul behind Magnatune launched a new book-sharing site, BookMooch. The idea is that you get to share books for free, using a system of rewards based on a scale of points: you get a point when sending a book (two points from outside the U.S. of A.), and "spend" a point when receiving one. Nice and easy, we'll see how well it works. The guy is technically sound (the site can be translated directly by users, à la wiki, and the main engine is based on consuming the Amazon webservice), and he's business-smart enough to have led an independent DRM-free web music-store for years. As of now, it's experiencing some loading difficulties, probably due to the premature launch (the UI could use some polishing); hopefully they should be fixed soon.
If you are interested and decide to join (it's free!), check out my wishlist, and my inventory (aka the list of books I'm willing to give away).