You see, Hogfather, who wears a red robe, gives gifts to children on Hogswatch (the last day of the year), and whose sled is pulled by four pigs, has disappeared. DEATH has taken over because if people stop believing in Hogfather, he'll stop existing and then the sun will not come up on new year. DEATH gets his grand-daughter to help find out what happened and get Hogfather back. The professors at the University get involved, including HEX, the thinking machine, and find out why you never open doors that are marked to never be opened.
This is one of Pterry's really good books and although it's about belief, you don't need to have any specific belief to read it.