The Millennium Series referred to the Millennium Series: The Magic Apprentice, The Millennium Series: The Dark Apprentice, The Millennium Series: The Star Magician, The Millennium Series: The Imperial Magician, and The Millennium Series: The Rise of the Dragon. These five novels were written by the author JK Rowling. As JK Rowling was a well-known author, her novels were usually sold in e-books and online bookstores, as well as in physical bookstores. If you want to buy physical books of the Millennium series, you can go to your local physical bookstore or online bookstore to search. Some physical bookstores may offer e-books or online reading services when purchasing specific books so that readers can read novels more conveniently.