This book had a great concept, but never lived up to its promise. 
 In the dystopian world in which the book is set, a disease causes some dead to reanimate into something stronger, faster, better. The "Reboots," as they're called, aren't considered human, and are persecuted, thus they're all but enslaved into doing the dirty work---like being bounty hunters and killers for the living humans. They work for a special branch named HARC, all of the Reboots living together, training together, each assigned with a number that indicates how long they were dead. 
 In this world is Wren, the best of the best. She is number 178, meaning she spent that long being dead before she became a Reboot. She is strongest, toughest, the number one reboot; as such, she is assigned tough cases and acts as a trainer for those newly dead and risen. I fully expected to love her character; from the description, she was supposed to be kick-ass, rational, unemotional, never letting her feelings (because reboots don't have much) get in the way. 
 The longer a Reboot is dead, the stronger they become, and the less "human," they are. They are less emotional, less feeling, more undaunted. As such, most reboots are feared, and the ones with the highest numbers are feared even more. Lower numbers are considered disposable; they're too weak and emotional to be of any long-term use. Adult Reboots don't make it, and are assigned to be killed, since they're too unstable mentally. Only children and teens are trained as Reboots working for HARC. 
 It took awhile for this, the backbone and the background to be given. As the book starts, we are immersed into an action scene right away, and not much is explain until well into the book. By then, I was quite confused and frustrated at what wasn't presented to me, and I still had questions that were unanswered, like do Reboots continue aging after they die? Why is it emphasized that higher numbers are considered so freakish, so terrifying, when really...almost 3 hours of death? That's not that long. That's not enough time for severe decomposition, probably not even rigor mortis. There's no smell of death after only three hours in a normal environment, yet Wren is so self-conscious about her time after death and others are so afraid of her because of it. It would make more sense if she had been dead for three days before rebooting, not hours. 
 Anyway, into this scenario comes Callum. Number 22. He's practically useless. He can't fight. He won't eat meat (Reboots need a LOT of food and energy to survive and do their various ass-kicking assignments), pokes around at a few pieces of food like a runway model on a diet. 
 "Just eat it and quit bitching about it," Lissy snapped from a few seats down. 
 You tell him, Lissy! 
 He also giggles EVERY OTHER MOMENT. I swear that's all he does for the first half of the book, he laughs, he giggles, he smiles. He can't take anything seriously. He's not a prankster, but he brings to mind the type of person who just laughs their way through life, not taking anything seriously, even in the middle of a precarious situation. 
 My eyes shifted back to his face to see a smile spreading across his lips. Why was he smiling? This didn't seem like an appropriate time to be smiling. 
 And... 
 ...when he caught me looking a smile broke out on his face, followed by a pout. 
 He pouts, he smiles, his eyes sparkles, he winks. I want to strangle him. 
 Callum is afraid to fight, he's almost a pacifist, and given his low number, he is absolutely terrible at actually fighting and staying upright. He disobeys orders because he can't stomach the violence. He is the most useless piece of you-know-what, the dumbest, most TSTL male in YA I've ever read besides for Calder-the-terrible-merman in Lies Beneath. 
 Besides the confusing plot, this had the potential to be a great sci-fi dystopian novel, but nope. Callum completely wrecked it for me. He effectually ruined Wren for me, too. I could not understand how such a normally cold, unfeeling, butt-kicking Reboot like her could fall so easily and uncharacteristically for someone like Callum, particularly when she is inherently unfeeling and inhuman, at such a high Reboot number. I just don't buy the romance at all. 
 The rest of the book is comprised of a halfhearted plot about escaping the HARC project and involves a mysterious experiment being performed on the weaker Reboots. I just couldn't really follow the plot. The writing was alright, but not engrossing enough to hold my attention, and it took me well over a week, reading a little bit at a time before I could finish the book. I would not recommend this.