These voyeuristic tendencies must satisfy some kind of primal need since I think that most people are curious to see what others are thinking, feeling, doing. This explains the premise of Found Magazine.
"We collect found stuff: love letters, birthday cards, kids' homework, to-do lists, ticket stubs, poetry on napkins, doodles-- anything that gives a glimpse into someone else's life.Another site is called Forgotten Bookmarks. Michael works in a used book store and blogs about "the personal, funny, heartbreaking and weird things I find in those books."
Abe Books website has an article called 'Things Found in Books' and it lists things such as old baseball cards, a pinky ring, photos,etc. Very interesting reading.