Terry Pratchett's The Colour of Magic

The Discworld - a magical realm quite unlike, yet hauntingly familiar to, ours - has got it's 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 extremely rich, by local standards - tourist through one of the roughest cities in the multiverse. And secondly, the world is coming to an end. Never rains, eh?