On the Sensations of Tone as a Physiological Basis for the Theory of Music
Helmholtz's foundational treatise on acoustics, timbre, and the perception of tone.

Helmholtz’s foundational treatise on acoustics, timbre, resonance, and the physiological perception of tone. Year given is the original German publication; the linked copy is a later English edition. Harry Partch discovered a translation in 1923, an encounter he credited as the turn that led him to just intonation.