Nicholas Tam has written a very long essay on maps in fantasy novels — their design, their relationship to the text, their use to the reader. It’s definitely worth reading in full; here’s a piece:

So when we open up a novel to find a map, we can think of the map as an act of narration. But what kind of narration? Is it reliable narration or a deliberate misdirection? Is it omniscient knowledge, a complete (or strategically obscured) presentation of the world as the author knows it? Or is the map available to the characters in the text? If it is, then who drew up the map, and how did they have access to the information used to compose it? If it isn’t, then through what resources do the characters orient themselves in their own world? And finally, does anyone even bother to think about these questions before they sit down to place their woodlands and forts?

In the post that follows, I am going to informally sketch out a theory of fictional maps, which is to say that I will put up a lot of pretty pictures from novels and talk about why they are neat. There is likely some academic work on this somewhere — I would be astonished if there weren’t — but I’m not aware of any, and certainly nothing that has accounted for modern critical approaches to the history of cartography. Map history and the comparative study of commercial genre literature are niches within niches as it stands, and my aim is to entwine them together.

Meanwhile, I stumbled across the following exchange, dating from 2006: Johan Jönsson’s essay in Strange Horizons questioning the need for maps in fantasy novels, and Matthew Cheney’s reply.