Discworld: Book 9 - Eric (The Unseen University Collection)

Eric is the Discworld's only demonology hacker.
Pity he's not very good at it.
All he wants is his three wishes granted. Nothing fancy: to be immortal, to rule the world and have the most beautiful woman in the world fall madly in love with him. The usual stuff.
But instead of a tractable demon, Eric calls up Rincewind, the most incompetent wizard in the universe, and his extremely intractable and hostile travel accessory, the Luggage.
With them on his side, Eric's in for a ride through space and time that is bound to make him wish (quite fervently) again - this time that he'd never been born.