My favorite website/list-generator Buzzfeed just posted a list called "15 Curious Things Found in Library Books."
It's pretty great. It's true that you never know what's going to fall out of a library book (though the cash one still hasn't happened to me...). But none of the 15 items hold a candle to what happened to me on my third day of working as a school library assistant.
Two 5th-grade students asked for permission to go to the high school library to check out a book of poetry. They came back, Emily Dickinson book in hand, with completely red faces. They said they saw something they shouldn't have and I noticed a piece of paper sticking out of the book. I opened up to it and lo and behold, it's an ad for male enhancement complete with visuals! Genitals everywhere. EVERYWHERE. My first thought was "THIS WASN'T COVERED IN TRAINING!"
They were fine and just wanted the book, but yeah I wasn't prepared for that one. Have you ever found something completely unexpected in a library book?