I have waited for this book to be delivered a day after it came out but couldn't wait and read the digital version of it while my physical copy was on the way.  
Anyway!!! Amazing,  first and foremost the idea is great and completely unexpected,  given the setting I was expecting something more rooted in local folklore but the actual plot was much better.  And, it did use the setting, using the fact that the Appalachian mountains are old and their amazing geology. I have to say that it wasn't "scary" not like What feasts at night,but that one played at each and every fear hidden in my Balkan soul. 
What Stalks the Deep was tense, some parts made my skin crawl (seriously fuck spelunking and mines) but in the end it had a really satisfying ending.

Additionally: I hope we see John Ingold, Chemist,  again and 
please, please tell me this will not be the last time we get to see Alex Easton. Here's an idea! Let kas next adventure be in Finland.