The books are very juvenile...they are in the teen section at the library. But although the show has girls in high school I'd say its focus is for a more mature audience...the show is better and I can't say much about the books because I couldn't read past the first one!
The books are amazing. However the show is not going off the books completely so i would watch the show and then maybe read the books or vice vera. Book are amazing and you wont be disappointed in reading the books!