I had high hopes for Hazrat Inayat Khan's "modern classic". And it certainly had many inspiring ideas as well as interesting insights and beautiful passages. But ultimately this book is neither one I will come back to nor especially recommend. I think this disappointment mainly stems from the fact that I was looking for a book about music that happened to be written by a mystic, whereas Khan is a mystic who happens to write about music.
As a musician and musicologist himself, music is of course central to Inayat Khan's Sufi mysticism. This is, however, really only because music stands as the most obvious experience we have of the vibrations which are, for Inanyat Khan, the ultimate nature of the universe. Rhythm and tone are thus simply the most abstract form of a cycle, a succession of opposites, which in the end always symbolizes the synthesis of these opposites into a harmonious unity. Music expresses the divine harmony of the universe. It's one of the great wonders of the world that the formless and abstract world of music holds such intense emotional power for us. Here is a 'cosmic vibration' that we can feel directly, both in its intellectual and sensual beauty. In fact, given the Islamic penchant for a purely geometric art, I think one would naturally expect a similar use of music. After all, it's hard to think of a better or more moving metaphor for the non-idolatrous worship of the 'face' of God. Indeed, this seems to be precisely the Sufi position on the matter, even though apparently this makes them quite the minority in the Islamic world.
Now, I would hardly object either to mysticism or to the use of music as an analogy in a mystical context. Unfortunately, lifting music up to this metaphysical plane can tend to rob it of its specificity. This is one of the problems that besets any articulation of mysticism. Since all things and all practices converge on the ineffable divine, the distinctions between them tend to dissolve and the starting points to become relatively meaningless. This can be either feature or bug, depending on your perspective. On the one hand, the divine is always only one step away. On the other hand, every step we take becomes in some sense the same. From the latter perspective, mystical thinking can become a bit repetitive, and that is certainly one of the things that made this book less enjoyable. Part of this repetitiveness stems from the fact that it was not written as a book but is merely a collection of unrelated public lectures for general audiences that Inayat Khan gave in the mid 1920's. But part of it is inherent to the nature of his mystical beliefs, which center on the omnipresent, unified, and unique nature of God. Since all doors necessarily open to a single divinity, the particular resonance of music with that divine can fade into the background.