This book is the one that convinced me that SJGames had things really, thoroughly on the ball. Like most of their other sourcebooks, this book is the product of thorougly researching a subject and distilling out the information which is needed to write immersive scenarios and stories in that context.
The GURPS Religion book takes this task seriously enough that it is usable not just as a role-playing supplement but more generally as a handbook of comparative religion for storytellers. It has everything from roles of clergy to creation myths to the meaning of religious symbols, told from the perspective of how religions perceive themselves.
The one caveat is that this book will not walk you through the process of designing a religion for a campaign. While the chapters are essentially a checklist for any religion's design, each chapter contains a large reservoir of ideas rather than a step-by-step instruction list. For the experienced GM who wants a steady fount of inspiration, or for anyone who wants a good introduction to the varieties of religious symbolism, this book is a valuable tool.
Definitely one of the most useful items on my GM's workbench.