I probably would have rated this 4 stars or more if I had liked the characters and their dynamics a bit better.  The writing itself flowed well, the setting felt like a part of the story, the supernatural stuff was cool, and the mystery had an interesting twist and resolution. 

Isla in particular got on my nerves and I found her behavior and perspective a bit frustrating. I think I suffer sometimes from "first POV wins" where I tend to latch onto the first POV character I follow and then if I get a contrasting perspective it throws me off, which is why I usually don't enjoy the unreliable narrator trope. In this case the book starts in Thomas's perspective and I found him pretty sympathetic throughout, but when we get Isla's perspective she's mostly annoyed and disappointed in him all the time. From the text, I get the sense that the author thought very highly of her, but I think her positive attributes were told a lot more than shown. Spoiler nitpick that illustrates my point:

Thomas is the one who actually finds Kit and gets him out by himself despite the harrowing and difficult circumstances, and yet everyone says Isla is the one who really found him?

The insta-love between Isla and Tanis didn't work for me, again due to feeling too much like tell vs show in terms of how awesome and great they both were. Unfortunately, my dislike of Isla dragged the whole book down for me to the point I didn't enjoy it as much as I wanted to.