The Art of Natural Dressage

Hi AND people. I am doing a huge history project for school for extra history credit, and I decided to do it on the history of Classical Dressage/Horsemanship, and how it has shaped the horse world today. This will be a very in-depth paper, mainly based on the history of it, but also tying in points from our modern times too. And with any luck, getting some points connected to the world outside horses. I don't know exactly where to start, it is a very daunting task... But I thought perhaps some of you wonderful people would have an idea to books that may help; considering we most closely follow classical principles(unless I am mistaken, and for which I apologize). In addition to earning extra credit much needed for graduation, i figured i could learn a great deal about this topic that is much forgotten with the rise of more traditional dressage, it is very fascinating and I'm excited to learn so much more. I have a small list of books I THINK will help with this project, but any additional info, or links, or anything would be incredibly helpful I also hope to throw in some connection with brideless/natural dressage too if i can. Thanks everyone for any help