At first sight this book looks great. It is concise and gives a good introduction from the start to build Joomla extensions. It does not waste pages explaining how to program in SQL and PHP. You are supposed to have mastered that. The book has a major weak point. For the main part it just shows lots of code, but does not explain anything about the API classes and how they relate. After reading the book and trying to create your own extension, it feels like you have not understood anything really.
A second drawback is that the author seems not to bother about testing the sample code. It contains several very obvious bugs, eg using a functou JOutputFilter instead of JFilterOutput. This is annoying.
Summarized, the book may give you a small start but is insufficient for seriuous development. The sad thing is that the official Joomla documentation is in a very alpha stage of development.