The scheme is called 'Bookcrossing' www.bookcrossing.com , check it on wikipedia.
You register on the website and get labels to put inside the book, each book gets a number. Leave the book somewhere and log it with the website with booknumber, place left and date (Released). It is then Wild until someone finds it (Caught), they then leave the details on the website. Apart from leaving them somewhere, you can also pass it on to someone you meet or send it to someone anywhere in the world who may be looking for a copy. Using its number you can keep track of it via the website to show how many times it has been released and caught and where it has been. I have been meaning to try it for ages so may give it a go now and see what happens............ (yes I am a sad bookloving geek aren't I! Most of my books are kept in cages for me ONLY to look at, if you love them set them free!)
Wheaten