The Wall Street Journal has an article about things you find in second-hand books. I can’t quite manage that but I did once find a letter from Arthur Cayley (who invented group theory) to John Couch Adams (who nearly discovered the planet Neptune) about how they were going to rig that year’s elections to the Royal Astronomical Society (probably 1860 or so), tucked inside a journal in the University Library in Cambridge. I told the authorities and they filed it in the archives.