The Duchess (Montgomery/Taggert, #16)

paperback editions sharing this same ISBN can be found here and here

Claire Willoughby risked losing her millions in her inheritance if, as decreed by her grandfather, she did not wed an "acceptable" man. Harry Montgomery, the eleventh Duke of MacArran, seemed perfect. He owned a historical castle, he looked manly in a kilt, and he was as much a titled Scotsman as Bonnie Prince Charlie himself.

Their engagement announced, Claire's future as a duchess was assured --- and she set off with her family to meet the Montgomery clan in Scotland. Bramley Castle was a damp, chill place, overflowing with eccentric relatives. But there was also Trevelyan, a secretive, brooding man who lived in Bramley's ancient halls. Whoever he was, he wasn't at all like Harry: Trevelyan was the most exasperating, arrogant, know-it-all of a man Claire had ever met. And the most fascinating ...