Her Highness and the Highlander by Tracy Anne Warren

Princess Mercedes of Alden is journeying home from a young ladies academy in Scotland when her coach is set upon by highwaymen. Lost, bedraggled and alone, she seeks help at a nearby inn. But with no money, and looking no better than a common beggar, the townsfolk laugh at her far-fetched store and turn her away.

Ex-infantry officer and dispossessed laird Major Daniel MacKinnon is having a meal at the inn when the wet, mud-splattered young woman appears, claiming to be a royal princess. Daniel doesn't believe her wild tale any more than the others, but something about her urges him to indulge her. He agrees to serve as her bodyguard on her journey back to London.

As the dangers increase, so does their closeness, until the two of them must face the riskiest hazard of all--falling in love.