Horses are Made to be Horses
by Franz Mairinger is probably the best theory book I have ever read. Franz was a rider at the SRS before he moved to Australia. He was our national dressage coach for a while too. He just has a beautiful understanding of horses and dressage, and how to really connect. Training the Young Horse: The first two years
by Anthony Crossley is probably my go-to training book. He really nuts out the steps, and sets you up to succeed every inch of the way. The book covers two years of training, and in that two years, he takes you from a freshly backed youngster to medium level, which is about 3rd level in the US I think.