I think that you must be a devoted worshipper of Shiva to fully comprehend the essence of this book. This is not a scholarly work, it’s a complex compendium of Shaivism, its lore and practices. Highly informative and detailed, however not in an academic sense. You must “feel it” in order to understand it.
Frawley’s language is very exalted, lofty and “mystical”. There is no doubt that he meant to arouse deep religious feelings and devotion in the reader. Some parts of this book might appear elusive and vague if you are not a devotee, they are too abstract for a novice to grasp.
My favourite chapter was about the great cosmic duality of Shiva and Shakti and I will certainly reread it in the future. I feel like so many scholars fail to explain it in a simple and clear way, often overintellectualizing, overcomplicating or simply not getting it, yet Frawley managed to do it in a very refreshing and captivating manner. I recommend it to everyone interested.
In conclusion: if you are a Western reader interested in Hindu doctrine and beliefs (but not a practitioner!), I doubt you’ll get much out of this book. I encourage you to try, but you will probably want to go for more specific and academic literature. If you are a Shaivite or at least a very spiritual person leaning towards this path, I am sure it will greatly inspire you and make a wonderful read.