For Parents: some swearing, kissing, the violence isn't graphic but there are child soldiers, executions, and murder
Cassie has survived the first 4 waves of the alien invasion: light's out, surf's up, pestilence and the silencer. Seven billion people didn't, including her mother and father. Her five year old brother was taken away in the 4th wave. Now she waits for the 5th wave and the courage to keep the promise she made to her brother, that she'd come after him.
This is a brutal invasion story. These aliens know how to wipe out humanity and do so in sweeping waves, each more destructive than the last. Through Cassie's eyes we learn about the 4 previous waves, and through her eyes, and those of a few other characters, we slowly learn what the 5th and final wave is.
There's a fair amount of violence, particularly when the child soldiers come into the picture. But it's violence that serves a purpose and isn't graphically portrayed. This is a book that questions humanity. What will humans do to survive? What makes you human in the first place? The book doesn't answer these questions, making it a great jumping point for discussion.
The characters are believable, falling apart under the pressures of the new world and picking themselves up again because doing otherwise means death. This makes the book difficult to put down. The writing is intense and while you may see the revelation that is the 5th wave coming, it doesn't stop it from being horrifying in its implications.
The ending is very tense, though I'm not sure I believe the kids could accomplish everything they do, and it ignores the larger picture. But it does give a good closure for the book.
I highly recommend it.