Hogfather: Book 4 in the Death arc.
Death - Exploring the afterlife with DEATH himself... and his skeletal horse Binky... and his daughter. If mass extinction, end of world scenario’s and anthropomorphic personifications tackling human emotions sounds like your thing look no further.
Susan had never hung up a stocking . She'd never put a tooth under her pillow in the serious expectation that a dentally inclined fairy would turn up. It wasn't that her parents didn't believe in such things. They didn't need to believe in them. They know they existed. They just wished they didn't.
It’s the night before Hogswatch. And it’s too quiet.
Where is the big jolly fat man? There are those who believe and those who don't, but either way it’s not right to find Death creeping down chimneys and trying to say Ho Ho Ho. Superstition makes things work in Discworld, and undermining it can have Consequences, particularly on the last night of the year when the time is turning. Susan the gothic governess has got to sort everything out by morning, otherwise there won't be a morning. Ever again...
The 20th Discworld novel is a festive feast of darkness and Death (but with jolly robins and tinsel too). As they say: 'You'd better watch out...'
In the novel, the Auditors strike again by deciding to eliminate the Hogfather because he does not fit into their view of the universe. They meet with Lord Downey, head of the Assassin's Guild, and commission the services of Mr Teatime, whose particular brand of insane genius makes him an ideal candidate for the assassination of the Hogfather and other anthropomorphic personifications. Death decides to take over for the Hogfather in order to make people continue to believe in him, wearing a long red cloak and a beard, but things start to become complicated because he is taking the children's wishes too literally.
Meanwhile, Death's granddaughter Susan must find out what's happened to the real Hogfather. She visits his Castle of Bones only to find the hung-over Bilious, the "Oh God" of Hangovers, whom she rescues before the castle collapses due to the lack of belief. In an attempt to cure Bilious, Susan visits the Unseen University, where it is discovered that several of these small gods and beings are being created. The University's thinking machine, Hex, explains that there is 'spare belief' in the world – due to the absence of the Hogfather – which is being used to create them.
Susan and Bilious then travel to the land of the Tooth Fairy where they discover that Jonathan Teatime has 'killed' the Hogfather by collecting millions of children's teeth and using them to control the children, forcing them to stop believing in the Hogfather. Upon throwing the Assassin off the tower and apparently killing him, Susan clears the teeth away and brings back the Hogfather by rescuing him from the Auditors, who have taken the forms of dogs. They cannot return to their original state and so cannot stop themselves falling off a cliff.
Afterwards, Teatime tracks Susan to the Gaiters' nursery, but is killed by Susan using the nursery poker, which passes through Death because "it only kills monsters".