'Tied Up in Tinsel' was Ngaio Marsh's 27th Inspector Roderick Alleyn story and was first published in 1972.
Synopsis from amazon.co.uk: "Christmas time in an isolated country house and, following a flaming row in the kitchen, there's murder inside. When a much disliked visiting servant disappears without trace after playing Santa Claus, foul play is at once suspected -- and foul play it proves to be. Only suspicion falls not on the staff but on the guests, all so unimpeachably respectable that the very thought of murder in connection with any of them seems almost heresy. When Superintendent Roderick Alleyn returns unexpectedly from a trip to Australia, it is to find his beloved wife in the thick of an intriguing mystery..."