As part of this project, I am opening and inspecting a lot of books. It should come as no surprise that I occasionally come across an object that has been inadvertently left in the book. People use all manner of expedient items to mark their place. In my own experience, I've used playing cards, business cards, paper clips and sheets from notepads to mark my place. Twice in my life I've found dollar bills in books; and many years ago, while working in a public library in North Carolina, I discovered a cashier's check in the 4-digit $$ range left stuck in a book. We were able to determine to whom the check belonged and it was returned. The owner was both embarrassed and grateful.
Things I've found in books in my library in the course of this project:no money so far. Drat.
checkout slips (lots and lots of these)
a blank postcard
a sympathy card
interlibrary loan slips
a quality control card
a list of call numbers
a post-it with a comment on one of Nietzsche's works
an outline of a biography of Sigmund Freud
an iTunes gift card from Starbucks for a band I've never heard of
the pull-to-open portion of a tissue box top with "I love Jesus" inscribed on the reverse
a syllabus for an English Literature course
a bibliography on post-it notes
index card notes on the psychology of emotions
a boarding pass stub with an email address written on it
a receipt for overdue fines ($0.55) paid - but from another library in the area
an invoice for over $600 paid to a student - but from another library in the area
a note card with an encouraging message to the recipient
I am not finished with my survey by any stretch, so I may be updating this list if I come upon anything else unusual or amusing. Check back every so often if you're interested.
What I haven't found yet is an actual bookmark - a paper or cloth slip designed for marking a place in a book. Our library, like most, prints out all manner of bookmarks and makes them available for our students to take. I think I'll be an optimist and say that they are being kept and used in the student's own personal books, instead of being returned to the library with our books.