I do not like books that do not have happy endings - in fact I go out of my way to avoid them most of the time. I picked this one up figuring that it was YA it would wrap up nicely as all YA books seem to do. Don't walk into this book expecting a typical YA/teen book. With that said, this was beautifully written, and the characters were nicely developed, the struggles they faced were real and powerful. 
 This book takes place in two worlds, the human one, and then also an alternate dimension (for lack of a better term) where two types of creatures are at war; the worlds are connected through portholes which exist in various places around the world. 
 I don't want to rehash the plot, but I will say that if you are a fan of great stories then you should pick this up, it is a beautiful tale of love, loss, and war. The characters in this are great because there is no clear line between good and bad, there are characters that were pushed to do what many would consider "bad" things because of the circumstances around them (circumstances which you will not learn about until the end of the book - so hang in there until the end), and there are characters which we want to believe are good, but may not be what they seem. (I think that was the worst run-on that I've ever written but hopefully you got my point.) 
 I hate that the book ended in a "to be continued", especially after the last major bit of information we get (if you read the book, you will know what I mean). I prefer my books to have a complete beginning, middle, and end, even if they are meant to be a series (as I'm assuming this will be). While we do find out what has been going on in the fantasy world for the past 17 or so years, I felt as though we were just beginning the real story when the book ended. Admittedly we did find out what the black hand-prints were, etc. Karou's story (and her previous life), Akiva's story, and where the war came from, but all of that seemed to be like "prequel" information, and I wish the story had gone on to see what happens after everyone found out everything.