This is one of those classic middle books in a series - it's good, but generally it's better within the context of the series rather than standing alone. The last book is much better.
That said, this book is very, very good. I love how Carrie Ryan has taken this whole zombie idea and used it as a backdrop rather than putting it center stage. I'm much more interested in the characters. While there is a fair amount of action in this book, the relationships between the characters are really more the focus. Ryan has done an excellent job of exploring all the feelings teenagers experience. The characters are entirely relatable, even though it's obviously fiction. I will say this: the bad guys in her stories are REALLY bad. They're the villains rather than the mindless zombies who seem to have just gotten a bad break and can't help what they are.
I've really enjoyed this series. You won't be disappointed.