I'm not quite sure where to start on a review like this, so I'll just skip to the end: if you are seriously thinking of writing WoW add-ons, then make sure you read this book! Now that I've got that out of the way, let me go back and explain why.
There are three primary reasons why I would recommend this book to someone. First, the author of this book created Deadly Boss Mods, one of the most popular (and indispensable) add-ons available, which means that the knowledge he shares comes from experience. That experience comes in handy for navigating around those "gotchas" that have crept into the API over time. Second, the technical information he provides is both thorough and presented in an way that helps you understand the "why" and "how" of add-on development. For instance, the author waits until the end of the book to explain macros. This might seem a bit backwards until you read the rest of the book and realize just how much you need to know to make the most of the macro system. Third, this book is an excellent introduction to Lua, the scripting language used in WoW add-ons. The book provides a thorough understanding of Lua, meaning that the information you learn for writing WoW add-ons will be just as useful when you decide to write Lua-based add-ons for other applications (or when you just want to use Lua on its own).
Now, there were a few things about the book that I initially didn't like. First, the book is written in an informal, somewhat conversational style. As a result, I thought this was going to be a "For Dummies" type of book. After a few chapters, it becomes clear that this book is definitely not for dummies and the relaxed style ends up making the information very accessible. Second, one of the larger add-ons that the author builds is a "Texas Hold-Em" client/server. Since I had no intention of writing this kind of add-on (and Texas Hold-Em is a little dated), I thought I was in for several chapters of useless information. Again, after reading just a small way into that section, it became clear that the internal workings of the add-on apply to all large, multi-user add-ons. The "hold-em" part of the add-on ends up being entirely secondary. Do not skip over this add-on!
So if, after all of this, you are still thinking to yourself that you can just read some online resources, dissect a few existing mods, then sit down and write your own, think again. Source code will not provide the knowledge you need to write high-quality, professional mods. This book provides that knowledge.