I would like to say a few words about a book I read some time ago and which has helped me a great deal when explaining intricacies of riding to all levels of riders who are searching for answers...
Anatomy of Dressage is "a new translation of the classic work and provides riders with anatomical basis for correct riding. Only with a working knowledge of human anatomy can a rider fully understand the instructions given for correct position in the saddle and explanations of the movements."
I would recommend this book to anybody who wants to ride better and who wants to understand what really is required from the rider's body in order to match the movements of the horse. I read it more as a teaching manual and inspiration for explanations but it can as well be a precious self-help for a rider who trains without an instructor/trainer on regular basis.
I got interested in this book after seeing who wrote it - Heinrich and Volker Schusdziarra also wrote a book that I bought age 12 or 13 and which was the first one to make me realise how many trees is in the equestrian forest! The deeper you go the more secretive and complicated it gets.
That first book was in a form of an interview in between the authors (doctors and dressage riders) on various teachings of various equestrian masters, including one I was really into at the time - Wilhelm Müseler. I remember really struggling for years to understand what they were talking about!
I am glad I kept going back to it and finding a new version was excellent. Not only that I understand it now but I can implement it and help others to do the same.
Have a go, you won't regret it.