Before getting into the details of the book I should point out that Open Atrium is a first class example of a Drupal distribution, a specially customized installation profile for Drupal packaged up with modules, configs and data to meet specific needs. Working with a "distro" package is a great way to cut back on time with a complete solution rather than building the same tools from scratch. I've read through this book with an interest in picking apart Open Atrium to learn how to do my own distribution for Drupal.
The book covers the install, configuration and customization of this flavor of Drupal very well. The Open Atrium intranet tool is well suited to collaborative work within an organization or business, and the book covers those details very well. There is a tight walk through of configuring new work groups in chapter six that is even better than many of the online documentation for the vanilla Organic Groups module for Drupal, which is exactly what Open Atrium uses. The instruction is thoughtful and concise, people looking for an Open Atrium resource will likely find this book useful.
There could have been a better job of helping a reader from an organization visualize the power of the Open Atrium environment. The examples in the various chapters did not inspire me to solve my organization's problems with their tool, but despite these weak examples I can vouch for the power of the tools myself based on my personal experience as a developer. If you are reading this and trying to decide if Open Atrium would be a good tool for your office then take my word that it will be excellent even if this book doesn't help you envision it up front.
I do not recommend that new Drupal users pick up this book as a means of learning Drupal itself. The title and tagline of the book do not make it clear that Open Atrium is a custom installation of Drupal, I am concerned that new users would pick up this book because of the price and availability and get their new learning all mixed up. I also found a few errors, typos, etc, that one can generally attribute to the nature of a book about open source technology - the books are written quickly to keep up with the development cycles of their subjects and as a result may fall out of date very quickly or overlook some of the fine details. Packt Publishing produces a lot of books on the topic of Drupal and generally their authors do a pretty good job.