Anything from Vladimir Littauer written after 1935. Truly eye opening. These are the first books I go to when I have a problem. The Udo Burger. I just have his book on riding (The Way to Perfect Horsemanship), but it has helped me a lot. This is the second opinion that I treasure. Then Podhajsky (sp?). Tom Robert's books are also filled with good stuff.
I am going to look up Basic Dressage for North America, it sounds like a cool book! I love The Dressage Rider's Survival Guide by Margret A. Odgers. It's funny and informative