An Affair at Christmastide. Mary and Bright series, #1

Nothing ushers in the holidays like mistletoe. and murder. Widow Mary Tomlinson isn't looking for romance. At four and thirty, she's content enough to keep house for her brother in London, but when the opportunity to act as chaperone for her niece at a Christmastide house party comes her way, she jumps at it. Even more when she discovers the party is hosted by Viscount Stanwick, a man she used to fancy in her youth.
The fly in the ointment is his brother and the troubling attraction for him she can't quite ignore. Inspector Gabriel Bright is attending his brother's house party in the countryside to help his son enter society. He cares not for ton life or for settling down; he feels most vital while working cases for Bow Street, but when the butler turns up dead shortly before dinner one night and Gabriel's son is found with the murder weapon, he's thrown into action.
This plus an impromptu kiss with a woman in the stables that excites his imagination in fires his libido might prove the company not as dull as he thought. Possessed with organizational skills and an affinity for broken things, Mary soon is recruited to act as the inspector's assistant as they investigate. Heat and desire simmer between them, but the two have more in common than they think when secrets from their pasts come to light during the case.
If they wish to explore a life together beyond the boundaries of their torrid Christmastide affair-as well as solve the case-they'll need to stay one step ahead of the killer.