Bottom Line Up Front - The Angry GM was honest when he pitched this book: it's not necessarilly for his biggest fans, but it does seem like an excellent resource for the new Game Master (GM).
I've followed the Angry GM for several years so I actually found myself very slightly dissapointed when I got done with his book. This is a portable, condensed, collected version of many of the foundational GMing principles the Angry GM has discussed on his website. That description matches exactly what the Angry GM advertised on his Kickstarter. If you are familiar with the Angry GM and have read his most popular articles, you've read this book. If you are not familiar with the Angry GM, it contains some of the best GMing advice you will ever read told from the perspective of a cartoon character version of a GM.
Angry outlines how RPG works and how to get started in what can be a very complicated hobby, before moving on to how to get started as a GM with basic skills. He covers keeping your combats fast and fun, player agency, finding fun for your players, game structure, resolving player actions, narrative conflict, managing NPCs, managing your group and your game, and a host of other GMing skills.
Minor Nitpicks/Warnings: I've had some people say they can't read the Angry GM's material because his "tone" or "voice" is distracting. That "voice" has been toned down, but is still present here. The Angry GM used the artist Jon Mosley, who has done the majority of illustrations for his website. The art adds charm, but it also sometimes feels a little crude. I've liked Jon Mosley's art for the website, but not necessarily for the book (sorry Jon Mosley).
Conclusion: I want to give this book a 4.5/5. It's quality, but I already knew all this stuff. I'm rounding up to 5 for the target audience. If you're already a fan of the Angry GM, I actually would not recommend picking up this book (outside of supporting the Angry GM/Scott Rehm). I bought a copy for myself and my sister and I'll probably try to find someone to give my copy. I don't need it. If you're an experienced GM who hasn't read any of Angry's stuff, you should buy this book for a few new tips and then go check out his website. If you're trying to figure out how to get into RPGs and/or learn how to GM, this book is better than any other source of GMing advice I've ever read on the internet, and when you're done with the book and running awesome games, Angry has even more content.