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.