This is one of my favorite books! I haven't read the third one but just bought it because I know I will love it. It's about four orphans who get selected to go on a secret undercover mission to a "school" with very suspicious actions. If you love mysteries this is the book for you. I read it two years ago and still remember most of it because I loved it so much. Probably ages nine through fourteen would like it the most, but I did get my mom to read it and she enjoyed it too. The only reason I wouldn't say below nine is because it's not exactly short. Of course, it's definitely not Gone With the Wind. You will love it!!