I couldn't yet read the previous volumes of this great collection, but I sure will after reading this book. Volume III is a hybrid product, half-insider in the game industry, half-game master advices. Wolfgang's chapters are specially interesting: you get practically a full script of how to design and create your product (even the psychological side of the process is discussed). There are no magic formulas here, but the articles really make you stop and ponder about how to implement your ideas (with great tips, even if you're just a GM working your home campaign).
Almost half of the product deals with more traditional themes (encounter, hooks, campaign structure etc), useful for DMs and professionals alike.