Gerd Heuschmann, Tug of War

Describing the basic anatomy and physiology of the horse, this book by an experienced veterinarian identifies widely-used incorrect training methods—especially in dressage—that can undermine a horse's health and well-being. Starting with the question Who is responsible for this? the book looks at breeders, breed associations, instructors, judges, riders, spectators, and the governing bodies of horse sports. The hyperflexion issue, the discussion shows, is that training affects horses both for good and for ill, and riders should reject any methods that cause pain or fail to respect the mental habits and physiological needs of their animals.

Specifically for Icelandic Horses, if you want to know the difference between a leg mover and a back mover, this is the book for you.