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. By J.K. Rowling 450 million Rowling’s seventh and final volume, Harry Potter and the Deathly Hallows, became the fastest-selling book in history, with more than 11 million copies sold during the first 24 hours alone. By R.L. Stine 350 million This series of children’s horror fiction, released through Scholastic, sold millions of copies a week. With 62 books published from 1992 to 1997, the cash piled up. And that’s the way many other book series get on the bestselling list: the Perry Mason books have 82 installments and have sold 300 million copies; Berenstain Bears number more than 300, at 260 million sold; Sweet Valley High (400, 250 million); Nancy Drew (175, 200 million); the nonfiction self-help series Chicken Soup for the Soul (105, 130 million). By Beatrix Potter 150 million First he ate some lettuces and some French beans; and then he could eat whatever he wants, because he would never go hungry again, with 150 million copies sold of these six illustrated children’s books, published between 1902 and 1912. By J.R.R. Tolkien 150 million One-third of the 150 million were sold after the release of the first film in 2001. By C.S. Lewis 120 million Tolkien’s friend C.S. Lewis didn’t do so badly, either. By Stephenie Meyer 116 million The young-adult vampire saga has sold quickly, but its staying power is questionable. People will read Tolkien and Lewis for ages, but will future generations stick with Bella? By Laura Ingalls Wilder 60 million There were eight books by Wilder, published before her death in 1957. But four more were published posthumously, with the latest in 2006. By Stieg Larsson 53 million The Swedish girl got a tattoo, burned things down, kicked a hornet’s nest, and helped her author’s estate sell a lot of books. By Suzanne Collins 26 million The dystopian trilogy has already outsold its closest forebears, the more finely written His Dark Materials books by Philip Pullman, which has sold 15 million copies. By George R.R. Martin 15 million How about a series that’s not even concluded yet? No one knows when Martin will finish his game of thrones, but sales are guaranteed to be strong and steady for many years. By E.L. James 10 million Holy cow! “The sales velocity for Fifty Shades of Grey is unprecedented, with reader demand still growing,” Vintage Books said in a press release this week. It’s only been six weeks that the publishing house took on the previously self-published e-book. For the naughtiest bits, you don’t have to buy anything—read it here! Fifty Shades of Grey Fifty Shades Darker Fifty Shades Freed