Stross runs Christmas through the Laundry. Who's coming down the chimney? Is it Santa Claus? Heh. If you know Stross's Laundry-verse, you know the answer.
Bob forgot to put in for time off for the holidays (too busy recovering from the events in The Fuller Memorandum), so he's forced to work Christmas Eve as the Night Duty Officer. In essence, he's there to field any calls that come in regarding extra-dimensional nasties. We endure the limited budget office Christmas party, complete with a guest speaker by the name of Dr. Kringle, and walk with Bob as he patrols the spooky almost-Escher office building.
Recommended for Laundry-verse completists. Better than "Down on the Farm" but not as good (or as scary as) as "Equoid."
Be sure to save room for the mince pies.