Terry Pratchett's The Colour of Magic

The Discworld—a magical realm quite unlike, yet hauntingly familiar to, ours—has got its first tourist Twoflower. Rincewind, an inept ex-student wizard, is given the task of guiding Twoflower through the city state of Ankh-Morpork. Rincewind has two problems: firstly, as an expert coward, he doesn't feel he's the best person to guard a naive—and by local standards extremely rich—tourist through one of the roughest cities in the multiverse. And secondly, the world is coming to an end. Never rains, eh?