The big reveal of the first book—and if you haven't read it, why are you reading this?—is that the fifth wave of invasion is the children. These kids, varying in ages from a few years old to 20ish, are trained to kill and then set loose, believing that they're destroying alien-infested people. In reality, they're just killing humans.
Now, normally second books in a trilogy tend to be a bridge between the first and last with less action and more of a developmental plot. This book, however, has several important action items, one of which is Evan Walker finding his way to Cassie. It also includes another good reveal at the end. In fact, the reveal makes you reconsider everything you thought about the Others (aliens).
We focus less on Cassie in this book and more on all our other secondary characters, especially Marika (Ringer). I confess that I wasn't a huge Ringer fan in the first book or the first half of this book, but she starts to grow on you, especially as she's intricately tied to the second action item. In fact, it's through her that we learn the big twist (it's good!).
As I've come to expect from Rick Yancey, this book is both lyrical and thought-provoking. One of my favorite quotes deals with the truth:
"It isn't that the lies are too beautiful to resist. It's that the truth is too hideous to face."Doesn't this describe the real problem? The truth hurts and we're often to cowardly to face it. Anyhow, read the book. It's excellent.