What Stalks the Deep (Sworn Soldier #3) by T. Kingfisher

From Tor Nightfire: 

An instant New York Times, USA Today, and Indie bestseller!

The next installment in the New York Times bestselling Sworn Soldier series, featuring Alex Easton investigating the dark, mysterious depths of a coal mine in America

Alex Easton does not want to visit America.

They particularly do not want to visit an abandoned coal mine in West Virginia with a reputation for being haunted.

But when their old friend Dr. Denton summons them to help find his lost cousin—who went missing in that very mine—well, sometimes a sworn soldier has to do what a sworn soldier has to do...