The books are alright, ive read all of them, JK has clearly changed the writing to suit a much older target audience than the first couple books after they started becoming popular with a much wider audience.
Frankly i thought the storyline of the later books felt too influenced by market pressures, the film industries and other outside influences.
Imagine those great sci-fi or fantasy book series that were completely written from start to finish years before they were optioned for any kind of film adaptation. Theres usually a consistency in content/tone throughout and the author wrote however the author pleased.
But now since pretty much the entire series was optioned for film before the 4th book was released, ive noticed reading the newer ones that the plots seem to be purpose written for easier conversion into screenplays.
Maybe its my imagination, i dont know.
Ed