This book (and the Sherwod/Clark/Lynas philosophy) was developed in parallel to the Zachman Framework (unbeknownst to either groups). If you a familar with Zachman, you will note several consistancies here. Though some may clain this is only a conceptual read, there are many oppurtunities to take pieces of the book and apply it in daily architecture. For example, on page 88, it gives several examples of "Business Attributes" in identifying types of business drivers ranging from user, management, operational, risk management, legal/regulatory, technical strategies and business strategies attributes. Thinking these through (and identifying which key ones are important) early in the stages of security architecture help direct the design in the right way. Also, the book provides several real world examples to help illustrate the "whys".
I had the oppurtunity to attend training given by David Lynas on Enterprise Security Architecture. I would also recommend attending, as David walks through several exercises in how to apply this methodology.
In the end, if you are responsible for any security architecture, using the principles/concepts/methodologies in this book will assist in making more concious, sound, security decison making.