As this book is a satire of Christmas, I normally read it around Christmastime, but I just couldn't resist.
This book deals with the power of belief. When the Auditors, beings who run the universe (and, incidentally, hate all life), hire the Assassins to take out the Hogfather (the Discworld's version of Santa Claus), Death has to step in. Aside from the pure comedy of a skeleton dressed up in red robes and a fake beard (it worked for The Nightmare Before Christmas, after all), the role brings up some questions in Death's all-too-literal mind. Like "why do the rich people get tons of presents, when the poor need them more?" and "why does no one appreciate me at *my* job?"
Early in the delivery route, he comes across his granddaughter Susan, who is working as a governess for a lady who is a bit intimidated by the fact that Susan also happens to be a duchess. Susan is a no-nonsense kind of lady, a bit like Mary Poppins - from the books, not the movie. She keeps the monsters at bay with the fireplace poker and discourages any attempts by her charges to seem excessively cute. And she's very annoyed by her grandfather showing up. But she listens, and ends up going on a quest to find out what exactly has happened to the real Hogfather.
Her search will take her from the fabled Castle of Bones (no one ever said the Hogfather lived somewhere nice) to Unseen University; from Death's little cottage to the land of the Tooth Fairy. It will introduce her to random fairies created by spare belief and to a crazed assassin who thinks differrently than other people. And in the end, it dips into the deep roots of old beliefs (possibly only familiar to folklorists, but well-founded in research.)
And as Death says, it is important for people to believe in the little lies, so that they can learn to believe in the big ones, like justice.
Because if we believe, someday it might be true.