For some reason, the cover reminded me of the Partials. This book kept me captivated. I'm not sure which part I liked the best. They were all equally tense, although I might go with the ending, especially the climax. 
 The world building was interesting, although the Partials had a more thorough description. So, it wasn't vague, but it could've used some more details, I guess. But then, this book is part of a series, and there probably will be plenty of more worldbuilding in the next books. This book was pretty unique, too - especially with the premise of the lack of oxygen.