We like Schooling Young Horses by Werner Storl. Storl exudes disdain for the reader: "Most people get the wrong horse for the wrong reason." He's quite German, of course.
We also like On Horsemanship by Xenophon, but then how can one escape Xenophon? His best ideas are in everyone's dressage book.
I continue to think of the words of Antoine de Pluvinel: "I work primarily with the mind of the horse."
The Pony Club books are excellent for beginning riders. Every new rider, whether in Pony Club or not, should own all of them.