The hottest trend right now in publishing is…things that were written more than a half-century ago. This month, publishers have enjoyed an unexpected bonanza from the vault, starting with the release of Harper Lee’s late 1950s creation Go Set a Watchman, which sold more than 1.1 million copies in North America during its first week on sale and now has a whopping 3.3 million copies in print. On July 28th, Random House will issue Dr. Seuss’ What Pet Should I Get?, another Mad Men-era manuscript expected to be a blockbuster, with a million copies already printed. Of course, the “forgotten treasure” boom can’t last forever; there are only so many relics you can pull out of the archives.