This Gun for Hire — Jo Goodman

He's got a job to do...
Former army cavalryman Quill McKenna takes pride in protecting the most powerful man in Stonechurch, Colorado: Mr. Ramsey Stonechurch himself. But the mine owner has enemies, and after several threats on his life, mines, and family, Quill decides to hire someone to help guard the boss's daughter. Only problem is the uncontrollable attraction he feels toward the fiery-haired woman who takes the job.

...but she's a piece of work.
Calico Nash has more knowledge of scouting and shooting than cross-stitching, but she agrees to pose as Ann's private tutor while protecting her. But between her growing attraction to Quill and the escalating threats against the Stonechurches, Calico will soon have a choice to make -- hang on to her hard-won independence or put her faith in Quill to create the kind of happy ending she never imagined...