I don't know what it is about this book and it's predecessor, but I absolutely adore them. 
 "The funny thing about armor is that it doesn't just keep other people out. It keeps us in. We build it up around us, not realizing that we're trapping ourselves." 
 I think it might have something to do with the fact that you can really find a little bit of everything in this book. There is a mystery to be solved with sufficient suspense to keep you reading later than you should. There is an interesting 'world' or rather aspect to be explored and explained that is so very unique. There are so many complex relationships in this book - like Mac and her parents who have all been grieving in their own ways and didn't lean on one another and as such there is now a void between them. There is a heart tugging love interest - with no instalove - who is adorable and the dynamic is hilarious and achingly sweet. (Okay, if I am honest I love love love Wesley Ayers, he's a major reason why I enjoy the series so much). There is exploration of people and how they read one another, how they react to pain and crises in their lives and how it changes them. 
 The way Schwab has written it makes all the emotions tangible for the reader, and I am left all achey and a little hollow inside. And I already kind of want to read both books again. 
 Also, I am still a major fan of the flashbacks. 
 "He leans in, resting his weathered hand on the bed. "Treat all the bad things like dreams, Kenzie. That way, no matter how scary or dark they get, you just have to survive until you wake up." 
 Mac is not a perfect main character. She is hurt and broken and she makes many decisions that will just have you going "What? REALLY? WHY?" and that makes it real. But she is scrupulous and sticks to her guns. There was good character growth for her in this book, she overcame a lot of things that were weighing her down. 
 "Caring about someone is scary, Mac. I know. Especially when you've lost people. It's easy to think it's not worth it. It's easy to think life will hurt less if you don't. But it's not life unless you care about it." 
 The side characters were all well defined and also had their own problems and were just well written, which always makes a book better! 
 Now, I have seen contradicting statements that state this is a duology and not a trilogy even though the Returned seems to be real..... So I am now going to go frantically find out!