Dressage
It literally is the best book written on riding I have yet come across. Ironic, isn't it, that I am not an English rider, but a Western rider and still found this book to be of great help. Yes, help. It describes in graphic detail what you should really understand before even thinking about mounting a horse, plus tips and guidelines on what you should know when you do mount a horse.
I really did find this book invaluable. I have owned it for years and I'm still learning from it. I find it absolutely great for references.
I would highly recommend this book to anyone who rides horses, and to those who don't, just so they can truly understand the significance of horse intelligence and human/horse companionship.