I love to find things left in books, and so I often leave things in them. I use receipts or pictures or ticket stubs for bookmarks, and then leave them to be found and remembered.
This morning at my parents' house, I took Petr Beckmann's A History of π (PI) of the shelf in my old room and started to read a little. A printed travel itinerary fell out, not for me, but for a friend. Apparently, my friend flew from Hartford to Sarasota via Atlanta on a certain day in the spring of 1998.
Perhaps I had lent him the book, and perhaps he uses the same bookmarking procedure that I do.