I just came upon what looks to be a very cool tracking site.
It operates much like Geocaching, except for instead of hunting for treasures, one can hunt for books.  If I am correct, instead of throwing away books they no longer wish to keep, book owners can place their book somewhere to be "found" by another person.  The cool part is the tracking---you can see where in the world your book goes!  Very, very cool.

Bookcrossing.com