I think one of the most overrated books, or series of books, is the Twilight series. The books are intended for a young adult audience, but I think young adults can do better. The writing is mediocre and the characters are predictable and one-dimensional.
The fact that the main female character needs to be rescued by male characters throughout the entire series also does nothing to help young women see themselves as masters of their own destinies. The fact that these books were turned into movies is confusing to me. There was nothing about the books or the writing that made me want to see the characters on a screen.