A best-selling book series often leads to successful movies due to the publicity the book has already gained for itself. As a result, production companies choose to adapt popular books into movies, and occasionally even fight for filming rights before the book is even published - as was the case for Margot Lee Shetterly's "Hidden Figures: The American Dream and the Untold Story of the Black Women Mathematicians Who Helped Win the Space Race." J.K. Rowling's "Harry Potter" series remains the highest grossing of all time, with 500 million copies sold in 80 different languages by 2018. These sales were undoubtedly affected by the extremely successful Harry Potter films as well.
[P1] A movie being based on a popular book increases its chances of being popular.