I purchased this book despite at least one scathing reviewer's criticism. The Author states specifically in the preface, "This book is intended to be a lighter read for programmers, embedded product designers, System-on-a-Chip (SOC) engineers, electronics enthusiasts, academic researchers, and others with some experience of microcontrollers or microprocessors who are investigating the Cortex-M3 processor." I interpret that to mean the book has a wide audience in mind, some with less experience than others. That said, I found this book to be on par with those written by various authors for the Newness series on Microchips products
The book has a good overview of the architecture without focusing on any one vendor's implementation. However, I felt that I would have liked to see a brief comparison of some of the various vendor offerings and what the key architectural components were implemented or left out. It can be argued that a vendor data sheet is the appropriate place for such information; just be aware this book is a general survey. The discussion in the back of the book on the Keil and GNU tool chains turned out to be non-essential for myself as I am using IAR's tool chain. Your mileage may vary.
Reason's to purchase this book, in my opinion, are to understand some of the background of the ARM architecture (not a complete review, but enough to understand where the Coretex-M3 fits in to the rest of the ARM series and the odd nomenclature of the ARM series) as well as an overview of how the architecture is put together. Then, a specific vendor's implementation of the ARMv7 can be pursued with a better understanding of what the vendor has implemented and how to use the implementation.
(Added 1/20/2009)
Note that with the introduction of Atmel's Cortex M3 product line, the popularity of the Luminary/TI Cortex M3 series, and others, this book is one of the few or, perhaps the only, books on the market that helps individuals new to the Cortex M3 get up to speed on the architecture and programming of the Cortex M3.