The erotic 'Fifty Shades' trilogy is about to sell over 20 million copies in the U.S., and the explosive demand shows no sign of slowing down. How does it stack up against other bestselling book series of all time? It’s notoriously tough to track total sales over the years, and older titles in the public domain can be sold by many publishers. Keep in mind we’re not counting single books here, and reference books with installments every year—the Guinness Book of World Records or the World Almanac, for example—are a different game. But here are some of the most popular book series of all time.