I’ve said it before, but we find a lot of bookmarks in books that are returned or donated. Sometimes, they are actual bookmarks. Most times, we find things like telephone bills, grocery receipts and old photographs (those ones are the most heartbreaking….we can’t always identify the people in them). Recently, someone even returned a book with a DVD containing photos from a special baptism. (If it’s yours, please let us know! We still have it!).
Our own collection of found bookmarks
Often, we can contact the person who last had the book and get the bookmark returned, but sometimes it’s more difficult. Especially when books are donated. And there’s an entire website (and book) devoted to things people leave in donated books. It’s called Forgotten Bookmarks, and the bookseller carefully documents each strange and interesting item they find, including what book they found it in.
For example:
Vintage trading cards
A Handwritten set of dinner topics
Maybe it doesn’t say much about the people who leave things in books, but it’s an interesting site and worth a few minutes to look around. At least it’ll give you something to think about the next time you pick up a book and pop something inside to mark your place. Just don’t forget to take it out!
Tales From An Open Book 
I like finding bookmarks or writings in books I borrow from a library or buy in a secondhand bookstore — it’s kind of fun to imagine what life the previous person was living.
I agree!