The Key to the Highlands by Blanche Dabney

Sent back through time, one woman’s life changes forever when she meets the handsome Laird of the MacGregors.

Author Rose Winter has made a career out of writing about medieval Scotland. She never dreamed her knowledge would one day be put to the ultimate test when she slips back through time. Rescued from an English dungeon by a burly Highlander, Rose realizes that not only is she an actual time traveler, she’s also falling for an actual Scottish Laird.
Lennox MacGregor is fighting on two fronts. One, to stop a cruel English Lord invading his country. Two, to resist his desire for the strange woman who seems to know more about his clan than he does.
As dark magic chokes the Highlands, Lennox and Rose must find a way to put aside their mutual attraction if they’re to track down the one thing that can save the clan from ruin, the mysterious key that’s been missing for hundreds of years, the Key to the Highlands.