Hogfather

'Humans need fantasy to be human. To be the place where the falling angel meets the rising ape.'
'Twas the night before Hogswatch and all through the house . . . something was missing.
Superstition makes things work in the Discworld and undermining it can have consequences. When Death realizes that belief in the Hogfather is dangerously low, he decides to take on the job. But it's just not right to find a seven-foot skeleton creeping down your chimney and trying to say 'ho, ho, ho'.
It's the last night of the year, the time is turning, and if Susan, gothic governess and Death's granddaughter, doesn't sort everything out by morning, there won't be a morning. Ever again . . .
Hogfather is the fourth book in the Death series, but you can read the Discworld novels in any order.