Bookcrossing is a social network that I have an account with. It is a community for booklovers and booksharers. Basically Bookcrossing comes down to labeling a book (having registered it online with Bookcrossing), sharing a book (by leaving it in a public place somewhere which has been logged at Bookcrossing) and then tracking that book online as it continues to be shared in the Bookcrossing way. This can be a great way to get rid of those books you no longer need or want and passing them on to others.
Get into the Bookcrossing community at
http://www.bookcrossing.com/