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.

The GM section at the end of Electric Bastionland is, for my money, the single best collection of GMing advice you can get a hold of.

Most of it (maybe all?) is also free on Chris McDowell's blog but having it curated and in print is great.

The two I would recommend are both centered around GM prep:

• The Lazy Dungeon Master by Mike Shea (@slyflourish@ttrpg.network , @slyflourish@chirp.enworld.org )
• Never Unprepared by Phil Vecchione (@dnaphil@dice.camp)

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

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.

The Conan books by Robert E. Howard; I know they're not RPG books but hear me out.

Robert Howard can fit a huge Conan adventure into the smallest space possible. Opening one story is like looking in the Tardis - Conan will seduce a queen, she'll get kidnapped, he'll ride vast distances, fight barbarians, then fight wizards, save her, then fight more barbarians and it's all in under 300 pages. The man was a genius when it comes to pacing which is a hefty problem for me and a lot if DMs I know.

Also Dungeoncraft on youtube.

@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

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.

Justin Alexander has a book on GMing coming out this year that I think will be very good. His blog is a wealth of good information

@Apex Urban Shadows and Dungeonworld for campaign structures Storms and Fronts respectively.

The Lazy DM series is a game changer, Return of The Lazy Dungeon Master in particular will revolutionise your dming.

@Apex Greg Stolze's How To Run A Roleplaying Game is pretty great bit for *new* GMs, though not so much for veterans:

https://gregstolze.itch.io/how-to-run-roleplaying-games

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.