Getting right down to the core elements of rider position, effect and influence, Anatomy of Dressage by Heinrich Schusdziarra and Volker Schusdziarra is a staple of the dressage (or any!) rider to truly understand their body in the saddle and how best to use it to for desired effect. Originally translated from German and now co-published by the United States Dressage Federation, there is a lot of information to digest, but it was easy to follow and broken down into logical parts, making a truly fascinating read.
This takes an almost investigative look at human anatomy as it relates to dressage riding and I learned SO MUCH from this book. Suddenly, things that my coaches had tried to explain were making sense. Between dissecting the half-halt, execution of aids, and position, this has it all; it truly is one of those essential reads. It is a short book absolutely packed with all the ohs and ahs you could imagine. Highly recommended!