I am in the bangor public library right now, and I was thinking it could be a cool marketing campaign to hide messages or something inside library books. I don't know for what this marketing tactic would work, but it seems sneaky and cool. Maybe not very successful though...you'd have to pick popular books, or books of a specific genre, appropriate to what u were marketing.

You know what's a really creative book? The Griffen & Sabine trilogy. There is actual letters inside envelopes in the book that you can pull out and read! (probably not here at the library though, because I bet people steal the letters.)