I was talking to a friend at a coffee shop the other day about libraries, and he remarked that books sometimes go through the Ann Arbor District Library so fast that they don't get thoroughly checked for bookmarks left by previous patrons. He's seen airline ticket stubs, note cards, and even a checkout slip that I accidentally left in a book.
I'd love it if every time I got a book there was some leftover of a previous reader - a few notes on a card telling me where the good pages are, or a bookmark at a particular fine illustration.
Here's a page of bookmarks from the Christchurch City Libraries in New Zealand. And here's to bookmarks that have a bit of blank space for patrons on them to take notes!