Secrets of Droon - book recommendation

Thanks to a friend, my son has discovered a cute series of books that has pulled him in. [For those that are against Harry Potter, this series is not for you either.] It's called "The Secrets of Droon" and is about a trio of friends who discover a magical world hidden under the stairs in one of their basements. Now I need to find out what the AR level is on those. :)