This is a difficult question to answer definitively as people's preferences can vary widely. However, some of the most popular book series ever written include the Harry Potter books by J.K. Rowling, the Chronicles of Narnia by C.S. Lewis, the Lord of the Rings by J.R.R. Tolkien, the Hunger Games by Suzanne Collins, and the A Song of Ice and Fire series by George R.R. Martin. These series have all sold millions of copies and have been adapted into successful movies and television shows, so it's safe to say they are some of the most popular book series of all time.
