The Tough Guide to Fantasyland by Diana Wynne Jones.
Any reader of epic fantasy or sword and sorcery will find it hilarious. Any would-be writer of epic fantasy or sword and sorcery should probably regard it as required reading. Indeed, in an online discussion, one writer told how she had heard of it, gone to Amazon and read a review that mentioned "STEW and other food cliches." Whereupon she placed her order and went back to her manuscript to edit it even before she got her copy. . . .
Hits lots of other topics. Like Magic and Swords. Enchantress. Children. The Barbarian Horde. Mines. Minstrels. And all sorts of delights.
She goes a bit overboard in places. Yes, if you go traveling in Fantasyland, you will eat a lot of Stew. It can be easily made in large batches and kept ready to serve whenever the wayfarers happen to stick their noses in the inn. And how much of the Ecology and Economics would be transparent to a traveler who does, after all, have the Dark Lord to think about.
But those are quibbles. Even where she goes overboard, many fantasy writers go overboard on the other side of the ship, so it may be useful as a corrective, even there.