Sworn Soldier: What Stalks the Deep, Book 3 by T. Kingfisher

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...