Amazon.com has a group of books called sleepover mysteries. They might be suitable.
There is also a set that was popular a few years ago, but the name has left my mind. They seemed so horrible, but there was always a reasonable explanation for the actions taken. Kids loved them and read one after another. They always wanted to discuss them. I think the author's last name started with S, but I am drawing a blank. Maybe this will trigger an answer from someone else