A book is not a film. Each format has strong and weak points, and as common as book-to-movie adaptations are, not every story is well-suited for both. Fans of the original books might bemoan when the movie version misses the point, but sometimes movies end up being the right way to tell a story that started in print.
We were alerted to this quick study from FiveThirtyEight last year that tried to figure out, statistically, what benefited and lost the most in the move to the screen. Using ratings from Metacritic and Goodreads (admittedly not a perfect comparison since Goodreads reviews come from fans), FiveThirtyEight zeroed in on books with the greatest quality disparity from their movies. Many of the most poorly rated movie adaptations are based on young adult or genre novels – which, again, fans may have inflated ratings for.
More interesting are the movies that significantly outperform their books: Up in the Air, Apocalypse Now, The Graduate, and others on the list were either adapted from books with middling ratings or reworked the premise for the zeitgeist (both, in the case of Up in the Air). Metropolis is a particularly interesting case, because the movie brought so much invention to film as a whole that the novel has been almost totally forgotten.
Both versions of Battlefield Earth, meanwhile, continue to be despised about equally.