When one buys a "gem" series book, it is to learn specific, useful techniques for implementing systems and algorithms. The "gems" themselves are the sought-after ideas and theories, formed into concrete examples that can be applied to ones needs.
Unfortunately, this book does not live up to its "gems" titling, only serving to dilute that designation with weak content. Many of the chapters read as if they were written by high school students that attended a seminar on "game engine issues" and were forced to write a paper to prove they paid attention. In other words, the chapters will talk about how you would go about doing something and what would be desired in the final product, rather than describing how it is actually done. As such, many of the chapters are extremely short, some as short as 2-3 pages, and have sparse generalization information in them--hardly "gems."
This book should be retitled, and directed towards people who want to read about general issues and ideas in game engines. It's not a "gem" book by any means, and you're not going to glean any sophisticated techniques from the authors. I also have to wonder about that "5 star" first review given to this book, whether it was a shill review, because I can't expect any professional keeping this book on hand, let alone as a reference. I wish I had spent more time looking at the "Look Inside" preview before laying my money down for this mediocre book.