Here is an article on cool stuff found in used books. (via kimbooktu)
The best part of working at the bookstore I used to was the finds. Every day in my first summer there my colleagues and I would find all kinds of things. Sometimes the donation boxes would reveal whole sagas; one box was filled with books on coping with the loss of a baby and later books on coming out and lesbian life with covers relying heavily on a aqua n' lavender color scheme. Inside one of those was a half-written letter lamenting a poorly-chosen expression of love and too much cocaine.
Some other cool finds:
1)Teenage love letters, in drafts
2)A b&w set of closeup photos of a man masturbating stuck in a copy of The Kingdom and The Power. They could be arranged as a flipbook.
3)Photographs from the early 20th century
4)Dollar dollar bills
What are your good finds?