Although I've only been using the book for a few days, and have learned a lot from it, I'm frustrated by mistakes in the code and lack of full documentation for the various methods that are used.
Background: I have a decent amount of experience with HTML/CSS and had set up a few Joomla sites before buying this book. I've used PHP before also, but for relatively simple things. I understand the basic precepts of OOP, but I am not a professional developer, so I thought (judging from the "who this book is for") this book would be a good way to start working on my own extensions. In some ways it is.
It does explain the basic things you need to do to build a new component, and uses a lot of Joomla functions that I'm sure will be useful, but it does not do a very good job of explaining how these functions/methods really work, what parameters they accept, etc.
The most frustrating part for me is that it seems there are some errors in the sample code. At first I thought this might be due to my own error, so I copied in the code samples available for download... there were still a few issues... so I installed the finished extension on a new install of Joomla, and it basically works - but still with a few issues.
In the end I'm sure the book will be useful, it's better than nothing, and I'm not aware of anything better - but something better could definitely be done.