Nineteen years later . . . Even as a new generation embraces the Harry Potter novels for the first time, J.K. Rowling's wizarding world continues to expand. Rowling herself has created a five-film spinoff, a two-part stage play, and an immersive online universe. The fictional sport of Quidditch now has a real-world counterpart, complete with an international governing body and a major league. Fans have adapted the series into role-playing games, crossover...