I am a great fan of the concept of bookcrossing, it really corresponds to an habit for me. Now what I find interesting is the notion of BookCrossing Zones. According to wikipedia:
Official BookCrossing Zones, which are sometimes called OBCZs or OBZs are located in places like Starbucks coffee shops, restaurants or other places where accessible to the public. These OBCZs refer to bookshelves placed there so that BookCrossers could catch or release books.
Look at this OBCZ World Map! Still a work in progress but very interesting.