Threshold by Jordan Hawk

Introverted scholar Percival Endicott Whyborne wants nothing more than a quiet life with his lover, private detective Griffin Flaherty. Unfortunately, Whyborne’s railroad tycoon father has other ideas: namely hiring Griffin to investigate mysterious events at a coal mine.

Whyborne, Griffin, and their friend Christine travel to Threshold Mountain, a place of dark legend long before the mine burrowed into its heart. A contingent of Pinkertons—including Griffin’s ex-lover Elliot—already guard the mine. But Griffin knows better than anyone just how unprepared the detectives are to face the otherworldly forces threatening them… Read more.