Scott Rehm (aka The Angry GM) taught me how to GM - first through the myriad of articles on his website, and again (in more organized and polished manner) in this book. His grasp of what makes RPGs work is unparalleled, and he consistently articulates a specific and consistent vision of how to run a game. He is probably the only GM-ing resource whose advice I don't think I've ever said "well that doesn't really make sense." With chapters on what an RPG even is, action adjudication, playing NPCs, types of players (and how to engage them), and even story structure, this book is a fantastic intro to tabletop RPGs.
My only criticism would be that Rehm uses a lot of video game analogies when he's explaining what RPGs are and how to run them. While this is fine for me (as someone who plays a good amount of video games), it doesn't work as well for folks who may be older or just not into video games. I had hoped to be able to gift this book to some friends and acquaintances I thought would enjoy RPGs, but those who don't play video games would probably find it hard to follow a good amount of the book.
That said, the above is a fairly minor quibble. This is a fantastic intro to RPGs and to GMing, and every GM who runs "traditional" RPGs (vs "narrative" RPGs like PbtA and FATE) should give it a read. Even those who only run narrative RPGs could probably get a lot of good mileage out of the chapters on story structure etc, just as GMs who run "traditional" RPGs can learn from the more narrative
Bottom line - if you want to know how to GM, read this book.
PS - for anyone who finds the "Angry" part of the persona off-putting, it's considerably toned down in this book compared to the website.