United We Spy (Gallagher Girls #6) by Ally Carter (Disney-Hyperion hardcover, 17 September 2013).
It's senior year for Gallagher girls Cammie, Macey, Bex, and Liz but they have other things on their minds. Cammie hasn't completely recovered from being imprisoned by the Circle of Cavan, but she and her friends think they've figured out what the creepy terrorist organization is planning.
Apart from Cammie's boyfriend Zach, and her mother (who happens to be the headmistress of The Gallagher Academy for Exceptional Young Women), it's really difficult to know whom to trust. Zach's mother is out for sure, but what about Preston Winters and his father, the US ambassador to Italy? And why does Max Edwards, the from Interpol keep popping up?
Close to graduation, they're being courted by international espionage organizations like the CIA, Interpol, and MI6. (Yes, they do have career fairs at the Academy!) But they can't even think about making decisions until they deal with the Circle.
The four girls are old enough, and experienced enough to assist with real investigations, and even to tackle them on their own, and their adventures are exciting and fast-paced.
Fans of this series must read this concluding installment, even though they'll be sorry to say goodbye to Cammie and her friends.
FTC Full Disclosure: I purchased this book.