Dressage is what remains of many different riding schools that brought their own styles into the mix. People have been riding horses for a long time, and good equitations was valuable, whether for war or for personal development (upper class, of course). Xenophon (greek horse master) wrote one of the oldest surviving texts that is still drawn upon for dressage concepts.
There is De Guernier (spell?), and Baucher and many others who wrote and taught.
What many people say is that dressage as it is practiced in modern competition has strayed from the original texts. That's where they say there is a "modern" and a "classical" dressage. Big debate nowadays.
You can form your own opinion after you read some of the various texts and watch some top competitors.
Sylvia Loch writes a good book on "Classical Dressage". Arthur Kottas has a more nuts and bolts book out that one might call modern dressage, but I dunno. I know that the old ways migt be unrecognizable to us. They rode in much harsher bits, often rode in saddles that look like bullfighting saddle of today, rode with more emphasis on collection and less on extension. They often rode one handed and carried a stick vertically in their hands. Check out some of those old engravings.