The Colour of Magic (Discworld Novels, Book 1)

Somewhere between thought and reality exists the Discworld, a magical world not totally unlike our own.
Except for the fact that it travels through space on the shoulders of four giant elephants who in turn stand on the shell of an astronomically huge star turtle, of course.

Rincewind is the world's worst wizard who has just been handed a very important job: to look after the world's first tourist, upon whose survival rests the peace and prosperity of the land. Unfortunately, their journey across the Disc includes facing robbers, monsters, mercenaries, and Death himself.

And the whole thing's just a game of the gods that might send them over the edge...

The Colour of Magic is the first book in the Wizards series, but you can read the Discworld novels in any order.