While the main Pathfinder book gives you pretty much everything you need to play or run a game, the Gamemastery Guide is dedicated to giving GMs a whole host of new tools to up their game to not just the next level, but a couple levels. All the way from the basics to NPCs to variant rules, it's got everything a GM needs.
From the post:
Unveiling the Pathfinder Second Edition Gamemastery Guide!
As the Game Master of a Pathfinder campaign, you’re the architect of your players’ adventures—a rewarding but sometimes challenging responsibility. With this book, your job just got a whole lot easier! Whether you’re building your own monsters and adventures, looking for new subsystems to play out cinematic chases or tense infiltrations, or simply want to speed up your game with scores of NPCs ready for action, the Gamemastery Guide has you covered!
The focus of every Pathfinder game is the players—their characters are the stars of the show and appear in every scene that plays out in game. But what about those scenes? What stories are being told? What old legends are being discovered, and what new ones are being forged? Who are the villains, the allies, the traitors, the lovers, the monsters, and the gods? Who runs the world? When you’re the Game Master, that’s all up to you!