That set-up for the sequel of The Hunger Games at the end of the first movie has apparently done wonders for sales of the book trilogy. Amazon says that it has become its best-selling series. The three books, it announced on Friday, have sold more copies than the seven books of the Harry Potter series. In a statement, Sara Nelson, editorial director of books and Kindle, said that "to achieve this result in just four years is a great testament to both the popularity of the work and, we think, the growth in reading digitally during that time." (Amazon also markets the popular Kindle digital reader.) The company did not disclose the actual number of copies it has sold of either series, but it did observe that they "got a big boost from films based off the books."