Another idea which may be of interest is BookCrossing:
http://www.bookcrossing.com/
It's a catch-and-release program for books.
Of course, I've only ever "released" seven books and none of them have been re-caught, but perhaps my release-spots weren't optimum.
Still, it might be of interest to some, with the added advantage that you can keep an eye out on the websites for books that have been released in your area, and go on a hunt for them.
Cheers,
Marc