Her Duke at Midnight by Christy Carlyle

When a young widow and an infamous duke meet at a country house party during the holiday season, the attraction that sparks between them takes them both by surprise.
Lady Cecily Wainwright has emerged from mourning eager for experiences she’s been sheltered from for a year and yearning for the passion she never experienced in her marriage. When her friend, Lady Fiona, invites her to a house party over the holiday season, she leaps at the opportunity to escape London and the watchful oversight of her brother-in-law. But on the journey to the countryside, her friend warns her to steer clear of one infamous duke who’s known for messy affairs and breaking ladies’ hearts.
Adam North, Duke of Everton, loathes his reputation as the “Heartbreak Duke” and has sworn off sensual entanglements for the time being. When he agrees to attend a holiday house party, his goal is to avoid wanton widows and eligible young ladies altogether. But soon after arriving, he encounters the spirited Lady Cecily Wainwright, who sparks attraction and curiosity. When the young widow asks him to instruct her in the ways of pleasure, Adam knows he should refuse. But Cecily is a temptation he cannot resist.

(This is a slightly expanded version of a story that previously appeared in the Duke in a Box anthology)