I had a chance to take a closer look at a new (and I believe the first one!) book about Firebug, written by Chandan Luthra and Deepak Mittal. This book focuses on Firebug 1.5, which is the current release on AMO.
After a brief introduction to Firebug and its history, the authors provide excellent and detailed description of all Firebug features (panel by panel) and so, the first half of the book represents a comprehensive reference guide for those who don't want to miss any tiny thing (gosh, I would love to have such info somehow directly accessible from within the FB UI).
The second half of the book was exciting for me. It starts with web page performance tuning and continues with description how to use Firebug for AJAX development. The last two chapters of this book are dedicated to Firebug extensions. You can find a list of some useful extensions as well as a great chapter about developing an extension of Firebug (including famous "Hello World!" example).
I think that there could have been more about how to effectively use Firebug in order to solve typical scenarios/issues, which developers meet in their daily work (improving download speed, profiling application logic, fixing design related bugs, learning unknown code & markup, etc) apart from listing all features.
Anyway, I highly recommend this book to everyone who is looking for complete and transparent overview of all Firebug features nicely explained and categorized. The book is worth reading!