The list of classic equitation books will never be complete, but here is a start:
Xenophon, On Horsemanship See the Project Gutenberg etext
Antoinine de Pluvinel, L'Instruction du Roi. Ca. 1666.
M. de la Gueriniere, Ecole de Cavalerie. France, 1733.
John Solomon Rarey, The Complete Horse Tamer, 1862. See his family's website for a complete etext
Edward L. Anderson and Price Collier, Riding and Driving. Macmillan, 1900.
Count de Souza, Elementary Equitation and Advanced Equitation
Piero Santini, The Forward Impulse. 1936.
Fort Riley Cavalry School, Horsemanship and Horsemastership
General Harry D. Chamberlin, Training Hunters, Jumpers and Hacks, 1934, and Riding and Schooling Horses, 1937.
Wilhelm Museler, Riding Logic. Simon & Schuster, ISBN: 0-671764-92-6 op
Colonel Alois Podhajsky, The Spanish Riding School, Vienna, 1948.
Margaret Cabell Self, Horsemastership. New York, 1952.