This is my first book on how to run a roleplaying game as the game master. Better late than never!
This book is not specific to any RPG system, but instead offers general advice for how to structure the game sessions, how to invent characters, settings, and adventures, and how to manage the sessions for maximum fun and creativity.
This book is full of advice from down in the trenches. I can see how certain sessions I've played in could have used gentle nudges from this book. How could I subtly introduce my GMs to it? Hmmm...
Among many other resources, there are endless tables of characteristics for character creation, buildings one might find in a settlement, roles for friendly GCs (gamemaster characters, ie NPCs)...
As someone exploring whether to start a game for my family, having played and enjoyed RPGs in the past, this was an excellent book for me. If you're new to the activity, you might want to try it a few times before reading, so you can understand what the book is trying to achieve.