I don't think either of these are perfect, but they both offer really good, actionable advice. The philosophies/systems described in these two don't naturally mesh, and I think that's a plus. Every GM needs to figure out for themselves how to prepare to run a game, because the things each GM needs are unique. I think having two books outline pretty drastically different approaches can help you triangulate your own needs and methods
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.
So much yes for blades in the dark, it has just the right amount of direction in a narrative game to help the people who are bad at improv. It's the perfect GM trainer tool, and just a great game.
People have already mentioned my two suggestions: 4E DMG2 and The Lazy Dungeon Master.
This isn't a specific book recommendation, but I would recommend reading a diverse range of RPG books, across a breadth of topics which interest you. In particular, reading the storytelling sections and how those guidelines interact with the delivery mechanics of the specific game can offer a new perspective on how to run your games.
For example, I enjoy horror roleplaying in particular, so books I may read are Call of Cthulhu for Cosmic Horror or the World of Darkness/Chronicles of Darkness books for Personal Horror. While I run Call of Cthulhu a lot more than World of Darkness, the tips on storytelling personal conflict from WoD offer some interesting insight into running sanity conflicts in CoC.
IMO one of the best books every GM should read is Your Best Game Ever. There are a lot of gems there that experienced GMs picked along the way, and even if you're experienced there is bound to be something interesting. One thing that I always remember from that book are tips on how to make your NPCs more memorable by giving them quirks like stutter or have a manerism when speaking.
Another cool upcoming book is The Weird, it's still not available even for backers but it's in the final stages so it should be available soon, but there is a preview available to see what to expect.
You'd have to track them down because they're a bit old, but the Central Casting trilogy of books I find very useful: Heroes of Legend, Heroes Now, and Heroes for Tomorrow. Basically they're filled with tables that can be used by any game to generate npcs, very detailed character backgrounds and personalities, random items or places, adventure ideas, just off the top of my head.
@Apex I must say I'm pretty impressed with #MothershipRPG's Warden's Operation Manual. Much of it isn't specific to Mothership and it contains solid and specific advice for most things I'd consider important for a #GM. #ttrpg