See this book? This is exactly the kind of book that we sometimes get in boxes of donated books to my library. The ones that make us say "Ew!" and run for the rubber gloves go straight into the dumpster. (Seriously - after a hurricane and subsequent flood, people were donating wet books to us. And that flood water was nasty sewage water - so you can imagine how the books smelled.)
I can't honestly say that we've been given any that were this bad, but some books have come close. Anyway, the above book sat in a closet in France for years and years. Ostensibly an old prayer book, it also had faint Greek markings running up and down the pages beneath the Christian prayer text. The family that owned the book took it to Christie's to see if it was worth anything, and it was auctioned off for two million dollars! Turned out that the Greek pages beneath the prayers were the only surviving copy of two manuscripts written by Archimedes.
This happened back in 1998, and it has taken nine years for researchers to uncover the text and diagrams hidden under the writing. And they have discovered that Archimedes "was working out principles that, centuries later, would form the heart of calculus and that he had a more sophisticated understanding of the concept of infinity than anyone had realized."
This Science News article will tell you much more about the story and the mathematical principles involved (which are all Greek to me, as I successfully managed to avoid calculus my entire life, ignoramus that I am). There are also some very cool photographs of the manuscript and how researchers managed to reveal words that a monk in need of paper erased so long ago.
I think I will be a little paranoid going through book donations from now on...