Murder on Mistletoe Lane / Clara McKenna.

Enhanced descriptions from Syndetics:

American heiress Stella Kendrick and her husband, British aristocrat Viscount "Lyndy" Lyndhurst, prepare to celebrate their first Christmas together as newlyweds in Clara McKenna's latest historical mystery set in England's New Forest region at the turn of the 20th century...

Taking on the responsibilities that come with being Lady Lyndhurst, Stella is eager to embrace yuletide traditions in the Edwardian English countryside and use her strong social influence for good. Her world becomes so consumed with starting a horse farm charity for the holidays that she barely notices the usual oddities attached to her upper-crust lifestyle. At least, not until items vanish from her bedroom and maligned housekeeper, Mrs. Nelson, becomes seriously ill-only to be found dead in the cold on Mistletoe Lane . . .

Cheery spirits are dashed following the sudden death, especially once Stella questions whether her own staff knows what-or who-killed the woman. Her suspicions mount when another person dies under strange circumstances during New Forest's annual Point‑to‑Point Boxing Day race. Then there's the case of Morrington Hall becoming plagued by false identities, secret affairs, and disgruntled employees . . .

Now, with two murders unfolding before their eyes in late December, Stella and Lyndy realize they can't fully trust anyone except for themselves while investigating. Because as disturbing answers come into focus, identifying the criminal responsible and surviving into the new year would be the greatest gift of the season . . .