Heard of François de la Gueriniére? Okay, that's good.
In case you're reading list is empty, add this to it – a rare translation Ecole de Cavalerie made by the late Captain William Frazer, which also includes notes from the Earl of Pembroke.
Google Books Link Ecole de Cavalerie : A Treatise Upon Horsemanship
Download the ebook version from ePUB
Also, it might be a good idea to print a copy so you can rearrange a few pages that aren't in order.