What’s the coolest thing you’ve ever found in a used book?

Recently I found a receipt in a used book from when the book was first purchased, way back in 1980. The receipt was from a long-closed bookstore on the other side of the country. I was able to find the owner’s obituary online. Tonight an old book I just bought yielded up a postcard from a bookstore in Cannes. It also looks old, but there’s no date so how old I can’t be sure. What goodies have you found?