MUSIC HARMONY BOOKS IN ENGLISH
Harmony 5th Edition by Walter Piston
20th Century Harmony 4th Edition by Vincent PersichettiHollywood Harmony by Frank Lehman
A Theory Of Harmony by Ernst Levy (recommended by Jacob Collier)
A Study of Modern Harmony by Rene Lenormand
The Unanswered Question – Six Harvard Talks by Leonard Bernstein
MUSIC HARMONY BOOKS IN FRENCH
Etude technique et stylistique de l’harmonie Moderne by Jean Doué
Etude sur l’Harmonie Moderne by René Lenormand
Traité de l’harmonie en 3 volumes by Charles Koechlin
Only available on IMSLP or in French libraries
Etude Comparée des Langages Harmoniques de Fauré et de Debussy by Françoise Gervais
This great book from 1971 published by Richard-Masse is unfortunately out of stock. Only available in French in libraries.