The magic in the books isn't really dark or, well, what I'd call realistic (not that I know much about that). Rowling has created her own world of fantasy-based magic, and it's really not that much different than in the movies, except that maybe one or two spells had their names changed for the movies for no apparent reason.

And if you liked the story-telling in the movies, dive right into the books. There is so much more depth there, especially in Prisoner of Azkaban, which, unfortunately, had to dump a lot of its backstory in order to keep running time down and keep the pace up.