I read the first of the books and felt it was a little young for me, so I never watched the show, but I have subsequently heard some things about it, so maybe I will start at the beginning and eventually get to season 6.
Lol not much like Scooby Doo. There's a group of teenage girls who are friends (though really bitchy to each other) and one of them disappears and there is a mystery about what happened to her. All of the girls start getting poison pen letters from someone calling themselves "A" who seems to know all their secrets.