The Ethics and Passions of Dressage
This book perfectly combats the "modern" theories on how dressage should be ridden by bringing to memory the time when the horse was trained out of necessity, yet the author does not extensively dwell on that time frame.
The memory created is enough for all who are well-versed in the sport to recall the truths that were formed by masters of the past (and then often subsequently buried by later "riders"), but it doesn't get bogged down in historic details.
He calls for a perfected technique of riding, born not out of technology or but out of a dance, which results in an exalted "partnership born of spirtied love, a love which is only heightened by understanding through discipline." I highly, highly suggest this book to anyone, whether they are just beginning in the art (or sport, take your pick), or whether they have been involved in the horse industry for decades.