The top-selling book series of all time is the "Harry Potter" series written by J.K. Rowling. This beloved fantasy series consists of seven books, starting with "Harry Potter and the Philosopher’s Stone" (released as "Sorcerer’s Stone" in the U.S.) and concluding with "Harry Potter and the Deathly Hallows." Since the first book was published in 1997, the series has captivated millions of readers worldwide, with over 500 million copies sold. The success of Harry Potter has also led to numerous adaptations, including blockbuster films and spin-off materials, transforming the franchise into a cultural phenomenon that continues to resonate with fans of all ages.