Kissing Christmas goodbye : an Agatha Raisin mystery / M.C. Beaton.

Enhanced descriptions from Syndetics:

Unlike quite a number of people, Agatha had not given up on Christmas. To have the perfect Christmas had been a childhood dream whilst surviving a rough upbringing in a Birmingham slum. Holly berries glistened, snow fell gently outside, and inside, all was Dickensian jollity. And in her dreams, James Lacey kissed her under the mistletoe, and, like a middle-aged sleeping beauty, she would awake to passion once more... . Agatha Raisin is bored. Her detective agency in the Cotswolds is thriving, but she'll scream if she has to deal with another missing cat or dog. Only two things seem to offer potential excitement: the upcoming Christmas festivities and her ex, James Lacey. This year she is sure that if she invites James to a really splendid, old-fashioned Christmas dinner, their love will rekindle like a warm Yule log. When a wealthy widow hires Agatha because she's convinced a member of her family is trying to kill her, Agatha is intrigued - especially when the widow drops dead after high tea at the manor house. Who in this rather sterile house, complete with fake family portraits, could have hated the old lady enough to poison her? Agatha sets out to find the murderer, all the while managing a pretty, teenage trainee who makes her feel old and planning for a picture-perfect Christmas, with James, all the trimmings, and perhaps even snow.