When aliens arrived on Earth, they had no interest in conquest or peace. They only wanted to kill everyone... and after four "waves" of destruction, they have pretty much succeeded. 
 Compared to a lot of the young-adult sci-fi, "The 5th Wave" is a bleak, genuinely harrowing story -- Rick Yancey throws his readers into a ruined world where humanity is almost extinct, and the few people who are left have been killing each other off. The constantly-splintering perspectives make it a little hard to follow, but the story is so horrifying, raw and tragic that it can be overlooked. 
 Cassie may be the last real human being on Earth. Ever since her father died and her brother was taken away, she has been wandering through the wasteland of what was once human civilization. Since the Fourth Wave, any human beings may actually be possessed by the aliens -- and so Cassie kills anyone she meets, before they have a chance to kill her. 
 But when she's badly injured by a sniper, Cassie finds herself rescued by the mysterious Evan, who nurses her back to health. In the meantime, her brother is whisked off to a military camp, where the boy Cassie was once infatuated with has transformed himself into the cold, guilt-ridden Private Zombie. But when Cassie discovers the truth about Evan, she finds that the 5th Wave is more terrible than anyone expected... 
 Rick Yancey clearly wanted "The 5th Wave" to be as grim and bleak as possible without actually featuring extinction. The first thirty chapters are entirely devoted to Cassie wandering through the wilderness, contemplating the extinction of the human species and thinking about the previous Waves. By the time she gets shot, it seems like things can't get any more depressing. 
 But once he introduces Evan, Yancey begins slowly revving up the story -- we have dual subplots, a military academy, a hint of romance (thank God, only a HINT) and a mysterious alien race whose nature is pretty nebulous. His prose is dense and slightly snarky, but with a melancholic undertone that really makes you feel like things might not turn out all right. It doesn't help that the aliens are so ridiculously powerful. 
 And the characters are beautifully written. Yes, there is a nebulous love triangle, but it comes and goes in the blink of an eye. The real focus here is on Cassie, who is a pretty strong heroine -- she was an absolutely ordinary teen girl until the Others come, and the loss of everything transforms her into an embittered, grim shell who clings to a few shreds of her past. 
 Evan's true nature is kind of obvious, but he is an intriguing character who transcends the cliches because... well, we aren't tortured by the cliched "humans are just so special and wonderful" garbage. And Ben is a great character -- his cowardice led to his sister's death, and now his feelings of guilt spur him to protect "Nugget" and help Cassie. 
 But Yancey does make a mistake with his writing: the bouncing perspectives. Most are first-person, with a few chapters in third-person. And the first-person perspective gets hideously confusing, since it switches between various people without warning -- including Ben, who isn't even really explored until the halfway point. It would have been better if the whole thing had been in third-person. 
 It's dark, it's bleak, and yet it's completely gripping. Despite the confusing first-person perspectives, "The 5th Wave" is a brilliant example of sci-fi for teens. Toss "The Host" aside and read something excellent.