Thomas Nicholson and Marc Sabat:
Fundamental Principles of Just Intonation and Microtonal Composition (2018)

Thomas Nicholson
Harmonic space calculator for HEJI and Johnston Notation (2018)

Larry Polansky:
A Few Words About Tuning

Marc Sabat:
Pantonality generalized: Ben Johnston’s artistic researches in extended just intonation (2014)
Shadings in the Chromatic Field (2012)

Catherine Lamb:
Interacting Spectra

Wolfgang von Schweinitz:
Die Urtonale Sonorität (2001)

Bob Gilmore:
Writings on microtonality and audio documentaries

Clarence Barlow:
On the Quantification of Harmony and Metre

Klaus Lang:
Auf Wohlklangswellen durch der Töne Meer (2001)

Georg Friedrich Haas:
Mikrotonalitäten (1999)
Jenseits der Zwölf Halbtöne (1999)

Kyle Gann:
La Monte Young’s The Well-Tuned Piano (1993)
Wolfgang von Schweinitz:
La Monte Young The Well-Tuned Piano 7-Limit Tuning (2006)

James Tenney:
A History of ‘Consonance’ and ‘Dissonance’ (1988)
John Cage and the Theory of Harmony (1983)

Harry Partch:
Genesis of a Music (1949)

Hermann von Helmholtz:
On the sensations of tone as a physiological basis for the theory of music (1862/1895)
Die Lehre von den Tonempfindungen als physiologische Grundlage für die Theorie der Musik (1862/1913)

Plainsound Music Edition
Cat Lamb: tuning resources
Joe Monzo: Tonalsoft Encylopedia
Xenharmonic wiki