*I skip around in this review and assume you know the basic synopsis* 
 I really enjoyed this book - though I don't usually gravitate toward YA - too juvenile - this never really had that feeling for me. I think it was because pretty immediately the emotional stakes in the book are high. 
 I very much enjoyed the MC's relationship with her father, and thought her relationship with his work partner was very believable. The plot was great and I thought it moved along at a nice clip. Once I started this book, I didn't put it down. I pulled an all-nighter for this one. 
 I only had a few major problems - and that's how the Demons came to the world - this fact seems very blurry to me. According to the lore of the book, one day demons just happened to show up, and chaos reigned until trappers and hunters started doing their thing. Which is fine - but I don't recall a specific day, time of year, or even in which decade it happened. I'm certain that day would be stuck in the social memory of everyone - even if it were in the distant past. Realistically, we remember catastrophic events - D-day, 9/11, etc. When a lot of people die, and the world seems forever changed - people remember the day. That this wasn't prominent (or at least introduced) seemed unrealistic - even in a teenagers mind. As a result, I felt fuzzy on when exactly the events are taking place (as far as their relationship to when the catastrophe happened). 
 My second major gripe: The only religion mentioned in this book is the catholic religion - which seemed to be doing quite well for itself post demons walking on earth. Since holy water is a well-used tool by everyone in warding demons away - does everyone now follow the catholic religion? How did demons physically being on the earth affect the other major religions? I don't need major details - but this would be important to the world, and should have been mentioned in my opinion. 
 My third gripe: Our MC has no close friend that's a girl. I found this extremely unrealistic. While I found it plausible she was close friends with a guy - I do feel there should have been a female counterpart. How many teens go through their lives with only one close friend and have that friend be male? I get that this book posits extenuating circumstances... but I still found it not likely. 
 On the plus side: 
 (view spoiler)[I really enjoyed the fact that demons had a counterpart and that they showed up in the book, eventually. It's also appropriate that they were frustratingly mysterious. (hide spoiler)] 
 I know I listed a lot of negatives, but that is not all this book has to offer, and as you can see from my 4 star rating it did a lot of other things right.