As a intermediate school teacher, I have got more reluctant readers to read with Robert Muchamore than any other author.
This book carries on where the last one in the series leaves off, with the kids being trained now and starting to look for new missions. It focuses mostly on the central character and his time in France sabotaging a German U-boat pen. Other characters from previous books are there as well, but they are more in the background.
Personally, I like this series as it is a lot more realistic in its feel. The level of knowledge of the author comes through in his creation of a realistic setting.
So if you want an action packed book with good historical background. Don't mind that any violence is honestly portrayed and not glorified or downplayed. Where believably human characters struggle to find their way. Then this is a book you will want to read.