Many years ago I read an article by Mary Wanless, and it transformed my thinking about my riding position, and how I could influence the horse. Recently, I have picked up the occasional ride again, and while I'm not competing, I wanted to ride in balance and in harmony, at least as much as my aging body can manage! This was fairly quick to read, but has some useful imagery to help you align your body to the best position. While it might not be the only book you'll need, it's a decent addition to any horse riders bookshelf.
Very interesting to read the actual text after I've been riding using these principles for the last year or so. It has vastly improved my riding and it was nice to read what she wrote and how my instructor and I have applied it to our riding. Highly recommend it for anyone interested in dressage.
I loved the details on helping a riders position. She has a way with words to just make your ride that much better after just reading this book. I can't wait to read her new book.