This book is concerned with the doctrinal beliefs of the practicing Zoroastrians examined within the frames of the Gothic principals. Variances in the current practice are examined.
Informative, but it would probably be fair to say that this book is written from the view of a practicing Zoroastrian seeking to explain his faith, and is not so much a historical or academic study of the religion.
If you really want to understand the historical context and evolution of Zoroastrian from an academic point of view, pick up Peter Clark's book or look for something by Mary Boyce.
This publication is a comprehensive introduction to Zarathustrian religion. It mainly focuses on explaining the core beliefs of the Mazdaists, as written down in Gathas, and pointing out the ways in which they were corrupted over time. An interesting read for all the students of comparative religion, or overly curious lunatics like myself.