The silver shooter / Erin Lindsey.

Enhanced descriptions from Syndetics:

It's the spring of 1887, and Rose Gallagher is finally coming into her own. She's the proud owner of a lovely little home near Washington Square, where she lives with her mother and friend, Pietro, and she's making a name for herself as a Pinkerton agent with a specialty in things . . . otherworldly, to say the least. Even she and her partner Thomas are managing to push aside romantic feelings for one another. Mostly.Things are almost too good to be true--so Rose is hardly surprised when Theodore Roosevelt descends on them like a storm cloud, hiring them for a mysterious job in the Dakota Territory. A series of strange occurrences in the Badlands surrounding his ranch has Roosevelt convinced that something supernatural is afoot. It began with livestock disappearing around the Little Missouri River, their bodies later discovered torn apart by something monstrously powerful. Animals have continued to turn up dead--and a few people too. Meanwhile, a successful prospector has gone missing, and rumors about his lost stash of gold have attracted treasure hunters from far and wide. But they keep disappearing, too. To top it all off, this past winter, a mysterious weather phenomenon wiped out millions of cattle.Roosevelt is beginning to believe that the land is cursed, and is sure something supernatural must be at play. So Rose and Thomas head West to hunt a monster . . . they can only hope it isn't something much worse.