The Countess and the Cavalier by Beverley Oakley

Forced to wed against her will at seventeen, beautiful Elizabeth Drummond’s loyalties are tested eight years later when her husband is imprisoned by the Royalist lover she was forced to surrender through duty.
Drummond Castle, home of staunch Puritan Silas Drummond and his beautiful wife Elizabeth, has been besieged by Royalist forces.
When the Commander of the hated King’s Men agrees to spare Silas’s life in return for a night with Elizabeth, she agrees.
Elizabeth may have been forced by her father to wed the cold-hearted Puritan, but an honourable wife will do whatever is within her power to save her husband.

But second-in-command Charles Trethveyan has other ideas. Eight years before, he and Elizabeth had been deeply in love. He’s not forgotten Elizabeth’s betrayal in relinquishing him to marry Silas.
When Elizabeth discovers that her former Cavalier lover has taken the place of his superior, and that she will spend the night with the man she’s never stopped loving, she realises she is treading a delicate tightrope.
Is Charles motivated by love or revenge? How much does Silas know and guess?
Whichever way she responds to Charles, her actions will have devastating consequences.
The Countess and the Cavalier is the fourth book in the Hearts in Hiding series but can be read on its own.