As a GM, your imagination helps drive the game. There are many ways to develop one’s imagination), but reading is certainly one of the best ways (if not the best way) to do so. So what do you read for the purposes of broadening your imagination?
When I was a child, I often heard the phrase “Garbage in, garbage out”. A lot of modern fantasy literature is garbage for the GM. While they may contain well-crafted stories, or well-realized worlds, they simply miss the heart of Dungeons and Dragons, even though it is often assumed that the modern fantasy reader loves D&D. For example, I tried to read The Wheel of Time. I got into the seventh book of the series and had to put it down.
Here is the problem with a lot of modern fantasy: The square peg of D&D does not fit nicely in the round hole of modern fantasy.
Look at the Appendix N books. Few of them would be 100% comfortable in the “Fantasy” section of a modern bookstore. D&D fits better into “weird” or “speculative” fiction category. Unfortunately, this is not an easy-to-categorize genre. It mixes horror, fantasy, and science fiction together (or maybe two out of the three). And, for good measure, throw in some myth elements into the mix!
But there is still a little hope with modern books! For example, look at a book like Somewhither. It has fantastic elements, science-fiction elements, and even some disturbing horror in it. It also, much like old-school D&D, mixes these elements into an incredible whole. If the author ever was a GM and invited me to his game, you better believe I would be there in a heartbeat.
Here is where the “garbage out” comes into play. If one reads nothing but modern fantasy, the campaigns and adventures that are created have a strong potential to be boring. When the party goes into the dungeon / ruined temple / undersea cavern / heart of the mysterious old forest*, there needs to be a sense that this place is different. The rules of reality may not exactly apply while you are there. Everything is a little more fantastic than the outside world.** In a lot of modern adventures, this is missing. I have been sorely disappointed in a lot of recently published adventures because they lack this “spark”.
TL;DR is you should read more “weird” or “speculative” fiction, particularly that which is on Appendix N list.
* For the purposes of the post, I am going to use the term “dungeon” loosely, to include all kinds of mysterious places that the party may visit.
** I would advocate for a somewhat “low magic” game world outside the dungeons. No cities with continual flame street lamps…unless the city is on a different plane.