In this context music is properly understood as an expression of and the motive for mathematical study. The "key" to unlocking this science comes from a study of the mathematical relationships between various musical notes.
McClain analyzes the Rg Veda, Babylonian, Greek (Plato), and New Testatment (Revelations) using this key, along with ancient definitions of numbers as "male", "female". The point here being that this "Pythagorean" analysis was fairly common and underlies much of literature of these and other cultures.
While McClain's analysis alone is worth at least a "4", I have given this book a lower grade.
The analysis is very detailed and heavily dependent on a knowledge of music, scales, etc. I found it difficult to follow in many parts.
The book's intended audience seems not to be the general public (like me) but rather a more learned audience. My guess is that this work either is his Ph D thesis or was based on it.
It's a pity that the work is not more accessible.
For those with an appropriate background, the book will yield a great deal more than I was able to wring from it.