At a Christie's auction in October 1998, a battered medieval manuscript sold for two million dollars to an anonymous bidder, who then turned it over to the Walters Art Museum in Baltimore for further study. The manuscript was a palimpsest-a book made from an earlier codex whose script had been scraped off and the pages used again. Behind the script of the thirteenth-century monk's prayer book, the palimpsest revealed the faint writing of a much older,...