Return of the Lazy Dungeon Master has already been mentioned a ton. It's amazing to give you a framework for session prep to build upon.

So let me add The Monsters Know what they are Doing. It's about giving enemies more tactics which are tailored to their stats and abilities. The book contains dnd 5e monsters but once you read it, you can apply the idea to any enemy on any game. I noticed I look at stat blocks differently after reading this book.

Lastly Blades in the Dark. Imo every GM should at some point run a short, several session adventure of this system. It's improv heavy and gives a lot of narrative control to the players. It changed my view on how to run several types of scenarios. And my ideas of what and how to prep. It's amazing how much I can leave to my players now in other systems too. Plus it's super fun.