At first, the references were frustratingly vague. Most modern books on horses in the middle ages focus on the role horses played in warfare. But then I discovered a reference that mentioned that the King of France had a 'ballet du cheval' (literally, a ballet of horses) performed for his wedding festival in 1612 (this date varies depending on your source). More digging revealed that there was in fact a book that contained information about said ballet, published in 1626 and written by one Monsieur Antoine de Pluvinel, who just so happened to be the King's riding master.
reprint of the original publication and lo, there in the back were not only woodcuts depicting the performance, but an actual description of the maneuvers performed by the riders. I expected it to be somewhat like the Spanish Riding School does today; instead, it's more of an English or Italian dance performed on horseback. Riders did things like take hands and turn around each other, shoulder-to-shoulder:
"Here three Horsemen faced three other Horsemen, doing courbettes one opposite the other and, as they came together, they took each other by the hand, two by two, in the form of a triangle, doing one volte and a half to the right; then changing hands and companion, they did a volte and a half to the left..."Fascinating!
Still more digging, and I discovered that the sheet music they used for the original performance in 1612 still existed, and in fact had been performed by a modern orchestra. I purchased a copy of the CD (yeah, before iTunes, I know), curious to hear if the music matched the footfalls for a horse's walk, trot, and canter like we try to do in modern freestyle dressage. The music is very stately and processional - you can hear a sample of it here, on Amazon. I don't think it would ride well for W/T/C, but if you're doing airs above the ground, it might work.
Unfortunately, the 1612 ballet du cheval was slightly "out of period" for the SCA, so I wanted to find proof that ballets du cheval were being performed before then. Besides, how did we get from using horses for transportation and warfare to the fantastic maneuvers performed in the early 1600s? I'd always heard that the high school dressage maneuvers like levade, capriole, and courbette were developed as war maneuvers, but how did we know that? Where was the documentation? And so I started digging deeper...