The Wolf of Carnoch Cross by Rebecca Ruger

Grace Geddes is out for revenge. If it’s the last thing she does, if it costs her every cherished penny or kills her, she will avenge her sister, assaulted and now lost forever. She sets off on her quest to find the man responsible, with only her furious determination and an aged soldier as her companions.
Ruairi mac Cailein wants only one thing: peace. He realizes as soon as he stumbles upon Grace Geddes that there would be no peace in her company, not with her misbegotten plans and her entrancing blue eyes. When she begs his help, offering him a ransom in gold, he refuses. He’s carved out a wee perfect life for himself and what few men remain of the once proud and mighty mac Cailean force, delivering his own brand of justice, accountable to none but himself; he wants no part of her foolish schemes.
Grace is terrified by the dangerously brutal warrior but imagines he might be just what she needs to exact her revenge. After she manages to convince him to help her, they set off on a journey both perilous and unexpected, each discovering that their long-held beliefs about life and love are challenged mightily by the other.