Among the Enemy is my personal favorite of the 7 books. Trey is a third child, an illegal. He is more on the shy side. Trey is very smart when it comes to books and liturature, but when it comes to getting involved with the population population police, well, thats where the excitement comes in.
Trey goes through a series a events, dangerous events leading him to find that "adventurous" side. So if you have a thing for action or myster, you should deffinately read Among the Enemy by Margaret Haddix.