Firstly, when I give this book 5 stars, I don't mean it in the same way that I mean it for the others. For me, 5 stars means "Meets all of my expectations", and this book does far more than that. This book is the cornerstone of Pathfinder. This book is a foundational resource to unlocking adventures here-to imagined.
Firstly, this book is not quite like the Pathfinder Bestiaries, which go into depth about specific creatures. This is a more esoteric volume, it is about creature templates, right out to taking two creatures and melding them together. The Pathfinder Bestiaries are add additional force, this adds exponential force, as the number of functional creatures that can be readily made with this book numbers well beyond what you will ever need.