This idea is somewhat a cross between Flat Stanley and geocaching. You log onto the BookCrossing.com site and register your book. Download its label (containing its ID number) or even just write the ID number on a bookmark & stick it in the book. Then "accidentally" leave it at the airport or at a restaurant when you leave the table.
The next person who picks it up, hopefully, will be intrigued enough to log onto BookCrossing.com and leave you a message in the book's online journal, telling you how much they, too, enjoyed reading it. Then, they, in turn, set it "free" again at some public venue.
Apparently there are already over 13,000 such "wild" books floating around the United States alone in this way, quite a few of which might be closer by than you think!
If you just can't bear to part with your favorite books, you can still participate by leaving journal entries in the "My Bookshelf" section so that people will still have the author & title of books you'd like to recommend.
Flat Stanley, for anyone who is not familiar with him, is an inveterate traveler drawn as a two dimensional Stanley (on any sheet of paper) by a school-age child (and/or an entire classroom of children) and then mailed to a specific friend or relative who is asked to take a picture of themselves with Stanley and tell how they entertained their "guest" during his visit. A Canadian teacher first came up with the idea and Stanley has had all kinds of adventures since. The kids who sends out a Stanley gets to track where Stanley goes and who he meets by return mail (and, yes, since Sept. 11 Flat Stanley is also sent out and tracked via email).
Geocaching, of course, is finding a treasure from its GPS coordinates as found on the geocaching Web site. BookCrossing.com is similar because you get feedback from people who find your book, just as from people who find your cache. Only in the case of BookCrossing.com, you get the feeling of having done a particular good deed.
For more information, see: