!til of book crossing -- "the practice of leaving a book in a public place to be picked up and read by others, who then do likewise."
There is a website (https://www.bookcrossing.com/) set up around the concept where you can register your book with an unique ID before releasing it into the wild. Then, when the book reaches someone they can enter the unique ID into the website to report that the book has been "captured". Thus, you can follow your book on its travels around the world. Of course, this is like a message in a bottle, and no one is obliged to report books that pass through them. Exciting!
My only qualms about the website are that I wonder what will happen to their unique IDs and capture reports if and when they shut down.