Alchemy

McClean, Adam, A Threefold Journey Through the Book of Lambspring, 2007? (pdf)

Math & Music

Fauvel, J.; Flood, R.; Wilson, R, Music and Mathematics, 2003 Oxford University Press (Amazon)

Music History & Biography

Levarie, Siegmund, Guillaume de Machaut, 1954 Sheed and Ward, NY (Amazon)

Philosophy

Davies, Stephan, Analytic Philosophy and Music, 2011? A brief historical survey of the philosophy of aesthetics particularly as it applies to music, from ancient to modern times. (pdf)