BookCrossing: n. the practice of leaving a book in a public place to be picked up and read by others, who then do likewise. (added to the Concise Oxford English Dictionary in August 2004)
I heard about this site and concept years ago, but remembered it over the holidays, during a conversation with my son and his friend, both avid readers and earth-friendly travelers. They are bicycling through Mexico, reading and releasing books. Now they know about this site and can spread the word. Check out the website at http://www.bookcrossing.com/ to read more about setting books free.