I haven't read the Horowitz, but a more musicological perspective, and one that is very clear in its explications of some of the math involved in the medieval period is Jamie James, The Music of the Spheres: Music, Science and the Natural Order of the Universe (1993). Have you read it? I'm not entirely convinced by his thesis of the return to world order in contemporary classical music, but I do think he does a good job of explaining some stuff.

Is it a HP fic you're working on? When/if I ever get to the planned ending towards which Phoenix Song is heading, I know Gary Tomlinson's Music and Renaissance Magic is going to become so important I might have to footnote it ;)