I am quite a fan of a website called Forgotten Bookmarks.
I have, though, myself found only library slips and bus tickets in the books so far. NB! now that paper tickets for bus have disappeared here, I am quite happy to find old bus tickets, as I can bring joy to people who collect those rarities by sending those away!
That is, until I gout this book:
… and found this boarding pass in it (so, as I got this book from UK, the book has been getting around quite a bit, it seems):
What is the most interesting item you have found in library books or in the ones bought used?