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

The next novella 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...