I like Kelley's books and the blurb was appealing so I knew I wanted to read this one. Unfortunately, it left me feeling lukewarm; I don't love it, I don't hate it, it's okay. Will I read the next one? Probably, but I won't be in a rush for it. 
 "Sea of Shadows" is for the most part a journey book; both a physical journey and an emotional journey. Ashyn and Moria undertake a journey to help the spirits which turns into separate journeys to find the emperor and warn him of the new danger to their land when the spirits and other beings attack. The sisters are identical twins physically but with individual yet cliched personalities. Moria is the outspoken, impulsive, loud, brave one and Ashyn is quiet, shy, and weaker. We're told that she needs protecting though why is not exactly clear, aside from her weak knife-throwing skills and the fact that she's quiet. Ashyn is not well developed but she's not as weak as Moria would have everyone think. Her character is weak due to lack of any actual depth, she's a stereotype. Moria was oftentimes obnoxious as was Gavril, the guard whom she travels with for much of the book. There's a budding romance there but they hide it in rudeness which slowly turns into understanding and compassion. Initially I disliked Gavril, then began to like him, and then there's a twist at the end that had me going "What?" I'm pretty sure that what we saw happen is not quite what really happened and I want to see how Armstrong handles it in the next book. 
 The emotional journey is less obvious but I suspect it will carry through the series. We get hints of emotional growth in Moria as she starts to understand Gavril and is a little less quick to jump to conclusions. "Sea of Shadows" has been described as epic fantasy and the journey theme fits in with that genre but it also has elements of horror and adventure and of course romance. 
 The most interesting character was Ronan, a thief who teams up with Ashyn for her journey. He makes no bones about who and what he is, he's smart and funny, he's a mix of predictable and the unexpected and there's a budding romance there also. He has more experience, in life overall, than Ashyn and he knows enough to go slowly with her. Although he can be a smart aleck, he's not cruel. He's the best developed character in this book and I hope that in book two we'll have more world building and character building, but also a faster pace to the story. This book sets up the story, if not the world, now we need to see depth and motion in the series. 
 Maybe in the next book we'll get answers to some or all of my questions; I spent a lot of time feeling confused: why have Seekers and Keepers? how and why do the spirits have so much importance? why is execution is outlawed? why does sorcery have a bad reputation? what other magic is there? how does the Japanese mythical creature kitsune tie into this world? (Gavril's surname is Kitsune but the world is not Japanese or at least not overtly.) Why are twins so rare and so feared? The world building was light which combined with the slow pacing of the book and the mostly flat characters made for a story that was disappointing and nothing wonderful. I loved the mythical beasts that we saw, they may have been my favorite part of the story. I also liked the different spin on zombies. The overall violence and gore may be too much for some young readers while others probably won't blink. I didn't expect how dark the story would be so at first it seemed too much. I would classify this as dark fantasy/horror. The world has overtones of different Asian and European cultures but doesn't identify with any one culture. 
 The series has potential but "Sea of Shadows" is a slow start. My fingers are crossed that the payoff in later books will be worth it.