Mort — Terry Pratchett

SOMETIMES EVEN DEATH NEEDS A HOLIDAY--and on the Discworld, the only Flat Earth in any universe, this skeletal keeper of all the hourglasses of life has suddenly realized the only way he'll ever get a chance for some time off is by taking on an apprentice. The unlikely lad he chooses is Mort, a boy with too many knees and elbows and a yen for the kind of knowledge he undoubtedly shouldn't have.

Though it doesn't take Mort too long to master walking through walls, learning to be objective is another matter entirely, especially when the life he's sent to collect is that of Keli, a beautiful young princess. When Mort kills the assassin instead of the lovely intended victim, he sets reality reeling. And with Death off on a tour of mortal pleasures, it's up to Mort, a rather incompetent wizard named Cutwell, and Death's adopted and highly willful daughter to try to mend the whole future course of History before the Discworld turns around....