Only the Good Spy Young is the Gallagher Girls book I'd like to call "know more about Zach GG book". As always, Ally Carter delivers a fast-paced, thrilling, adventurous, romantic and enigmatic story in this fourth installment of the GG series. 
 I cannot believe it's the fourth book already! It only feels like yesterday when I opened the first one and read about Cammie and her elite, spy school. I still feel a lot like there's more that we don't know though - about Cammie, her past, her father, and of course Zachary. But I have to say that there are some pretty juicy stuff we'd be (finally) privy to in this book and they're quite surprising. 
 I love that there's so much more mystery and action in this book than the previous ones. It's packed, fast-paced and there's so much going on in every page turned. I love how Cammie finally unravels bit by bit things about herself and her past she never knew before. We'll see a different, stronger, more grown up Cammie in this one and for that I came to adore her character more. Also we get to see Zach in a whole new light - that there's so much more to the cocky, tough guy exterior he chooses to show. It felt nice to see that for once, Zach was sharing things about himself voluntarily (although very reluctantly) and not hiding the truth with his sarcastic words and swoon-worthy smile. This books presents a vulnerable, soft side to him and I just fell in love with him even more! 
 This book will definitely not disappoint. I feel that it's cut too short though and that some things should've took more time to develop and unravel. But aside from that, I couldn't come up with anything more to say that I didn't like about the book. 
 Trust me, you wouldn't want to miss this!