Letterboxing is a really cool past-time that's been gaining in popularity lately. Generally speaking, it's like treasure hunting where the treasure is more in the experience than in what you find. You get a clue, solve the puzzle, oftentimes hike to the location, and discover a rubber stamp usually created by the person who left the letterbox, and stamp your own letterboxing journal. There's more involved, of course, but if you're really interested, you should get the book!
I first heard about letterboxing from a friend, and I wanted to see what it was all about, so naturally I bought a book on the subject. The Letterboxer's Companion is a great introduction to the hobby. It gives insight into the history of letterboxing (it's not a new thing), while also guiding the reader through finding letterboxes and creating their own. There are also chapters on creating one's own rubber stamps and the etiquette involved in the activity as well as a section dedicated to helpful internet resources.
As a reference book, Randy Hall has created a very thorough and readable guide.