Benson, D. J. Music: A Mathematical Offering. Cambridge: Cambridge UP, 2007. Music: A Mathematical Offering. University of Aberdeen, 14 Dec. 2008. Web. 13 Dec. 2014. <http://homepages.abdn.ac.uk/mth192/pages/html/music.pdf>.
One section starting on p.224 contains an overview of the mathematics of the scale and its history. The first sections contain useful information on mathematics applicable to acoustics, sound, etc. including calculus and frequency analysis. This mathematics is more advanced than most other sources I could find, so early sections were also useful.
Bartlett, David. "The Mathematics of Tuning and Temperament With Audio Examples." The Pyxidium. David Bartlett, 1998. Web. 16 Dec. 2014. <http://www.pyxidium.u-net.com/Acoustics/MusicMaths/MusicMaths.html>.
Background information that I needed to explore advanced resources. This source is easily accessible and covers the rudiments of pitch and frequency.
Grove, George, and Stanley Sadie. "Microtonal instruments." The New Grove Dictionary of Music and Musicians. Vol. 16. London: Macmillan, 1987. 617-25. Print.
Knowing about microtonal instruments lets me find out what notation was used to write music for them before the modern era.
Burkholder, J. P., Donald J. Grout, and Claude V. Palisca. A History of Western Music. 9th ed. New York: W. W. Norton, 2014. Print.