This is a great concept & service for our readers. BookMooch is an online book swapping web site that has 45,000 members and 750,000 books in its inventory. What goes on here is fairly straightforward. Users can give away books they've read in exchange for books they want to read -- all for no money, other than the shipping costs. That makes it easy to support your reading habit, and it saves a few trees. The inner-workings of the book swapping are explained here and here. Have a look.
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