There's a whole organization/hobby dedicated to this idea - "releasing books into the wild." http://www.bookcrossing.com/

It can be as simple as leaving a note, but they also have book stickers you can use to explain that the book is free, not lost, and outline the catch-and-release philosophy. The idea is to encourage the finder to keep passing the book on. For real hardcores, the site even has a system for tracking books that have been set free.

I don't do this formally, but it sounds like it could be entertaining.