If you have a pile of (non-library) books filling up your shelves that you have read and are perfectly good but you don't really feel a need to keep forever, you might be interested in Bookmooch. It's a book trading site that gives you points for uploading your inventory of tradeable books, and then lets you swap points for books (you "mooch" a book by requesting it from the site, which in turn notifies the book owner). Successful swaps beget more points, etc etc.
Point-based systems always have some tricky economics going on to make sure that you don't get too many points floating around or too few to match up with and encourage good behaviors. (See "Monetary Theory and the Great Capitol Hill Baby Sitting Co-op Crisis" for theory in action.) Otherwise this is set up very similar to la la (for CDs) in many ways.
On my wishlist for Bookmooch is some level of integration with Amazon or LibraryThing or any of the keep-my-wishlist sites that I know are out there - there's no particular reason to store that stuff twice.
Thanks to Jeff Ubois for pointing this out to me and introducing me to Bookmooch's John Buckman.
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