Only the Good Spy Young, by Ally Carter

I've been reading the Gallagher Girls spy stories (otherwise known as the books with the awesomest titles in the whole world) for a couple of years now, and while I did fall harder for Carter's new Heist Society series, I have always enjoyed them.

With this fourth book in the series, though, I think Carter has quite noticeably lifted her game. The earlier books were fun, frothy adventures about a spy school for girls, and while there were hints of darkness and seriousness they were often...