Into the Face of the Devil — John Rose Putnam

When Lacey showed up in the gold rush boom town of Hangtown where Tom helped in the cafe, he was smitten. About the only pretty girl in the whole of gold country she was looking for a Pa who'd come to the mines and disappeared. Tom had to help her, but as soon as he did somebody shot at him and things went south in a hurry. Then he learned that Lacey's pa was an important officer in the army who'd been working undercover in San Francisco during the Mexican War and he knew he'd stuck his nose deep into the evil skulduggery of some pretty bad men. If he didn't find her pa soon the next goose cooked would be his. That notion chilled him to the bone.