Little Bits of Paper in Books
I found this at PostSecret:

What a cack. It reminded me of something…
Last week I was reading a book at the library, and out fell a piece of paper with an email address on it… I’m gonna email that address, explain where I found it, and see what happens. Perhaps this could be one of those moments where something bizarre happens, or perhaps I could get an “Undeliverable” message back.
Either way, I’m game…. shall keep you posted…

So what happened with the email address?
Closer inspection of the email domain indicated the address was for a travel agent, and I don’t want spam aplenty, so no dice on the email front.