Hogfather by Raymond Briggs

When the mythical gift-bringer vanishes on the night of Hogswatch, Death steps into the role to keep the world's traditions and the fragile human belief that underpins reality intact. His no-nonsense granddaughter Susan investigates the disappearance, uncovering a sinister conspiracy involving impersonal cosmic forces and a ruthless assassin who would extinguish imagination. The collision of dry, cosmic logic and stubborn human warmth becomes a darkly comic, affectionate meditation on the power of stories, duty, and why myths matter.