A knowledge of this background really helps you to understand what you're trying to do. I would recommend the classical Greek book by Xenophon, books by Pluvinel, the Duke of New Castle, and Gueriniere. However, keep in mind that reading a book still won't help you do it yourself. You need expert, perceptive help on the ground. Riding is done on the horse, not at a desk.
Riding is body memory.
If you go to a clinic, it's a good idea to ask your instructor to come to it with you, for a couple of reasons. First, you can learn together. Also, that way he won't feel depressed that you've left and gone off to get some new ideas to make the training easier. There's a lot of ego involved. A good way to start using clinics is to audit them.