There are things you want to find in a library book, and things you don't. I am an avid reader of books, both library books and used books, and usually come across fairly mundane items - I'd say boarding passes are the most common in used books and grocery store receipts are the most common in library books. However I have come across photographs, letters, and currency.
I don't know how many of you remember this story from a few years back about a librarian in Santa Fe who came across the least popular item ever to be inserted into a book - a small envelope of smallpox scabs. Eeeuww and Arggh!