Even if this book is aimed to professionals/freelance (or wannabe) GDs, I bought it to foster my skills as an amateur designer and game master.
It provided a ton of valuable resources, ideas, techniques and tools for my belt.
If you're an experienced game master which wants to step in game design as an amateur this book can be a little overkill, so take it if you can afford the price or turn to the interweb.
If you plan to make the jump as a professinal, you'll find this book is a great kickstarter, covering all expects of the GD art, from worldbuilding to rules design to pitch elevation and how to meet publishers' tastes and needs.
Four stars only since a couple of essays felt like fillers to me.