tl;dr: highly recommended for anyone who has ever seens a horse and liked it!
As both a biologist and a professional equestrian I think I know quite a lot about horses. I have been learning about them from books, scientific articles, from a decade in vet school back in the day, from instructors and trainers and friends, and from horses themselves. Knowing the publisher and editor (COI note: I have published with them in the past), I expected the book to be very good. I did not expect that it would be so stunningly good! Yes, there is a lot of information there that I am familiar with, and all of it is presented beautifully, engagingly and accurately. Sure, if I was the editor, I may have changed a word here and there, perhaps a "talent" into "adaptation", a "developed" into "evolved", but then I check myself and pull back my inner scientist and accept that some of that wording, though scientifically not super-kosher, is accurate enough, and kinda poetic and inviting as well. The book certainly does not avoid the subject of evolution and does not have a feel of "are the Creationists going to react?" at all.
What surprised me was how much new I learned from the book. I learned more facts about the horse origins and evolution, ecology and natural history, anatomy and physiology, psychology and behavior, as well as about the ongoing history of the horse-human relationship, than I ever dreamed I would get out of this book. I guess my 20-year focus on mainly birds, and specific aspects of physiology and behavior (mostly daily and seasonal timing), made me miss all the exciting findings of the equine science of the past couple of decades. Importantly, some of the information is directly relevant to people like me who work with horses every day and teach the next generations of horsepeople. Some of it wil start getting incorporated into my teaching tomorrow!
For most of history, the main body part of the horse that people paid attention to were legs. The old adage "no feet no horse" reigned supreme. After all, horse does all of its work by moving with its legs.
The 20th century brought some changes. Some people started focusing too much on the head carriage, which brought in a whole host of training gear and methods, from side reins and draw reins to rollkur, into training of horses. Head carriage is supposed to be an indicator of good weight-carrying by a horse, not a means to that end. The backlash is now in full swing, with most of those methods and tools being regarded as anywhere between useless and harmful, even as horse abuse that is getting banned from competition grounds. Natural horsemanship (with all of its silly mystical excesses) is one of the over-reactions to this development. Exploration of bitless riding is another, much more sensible reaction. Some people are getting back to the focus on legs.
Others are focusing on the back instead, some, like Jean-Luc Cornille, going to the extreme (but backed by quite a lot of research and logic and demonstrable effects) of placing the back so much to the center of attention that many lamenesses as well as behavioral problems are now squarely blamed on the back problems (as opposed to reverse as many vets do). As it tends to happen when people disagree, there is too much anger and too much denying that the other guy may be also right about something. Just because you have a hammer and everything looks like a nail, does not mean that some things are not nails, and does not mean that some things have to be nails. A combined, cool-headed approach is probably the best - sometimes it's legs, sometimes it's back, and sometimes it's something third....
But there is another new branch in the equestrian training arising recently, based on scientific research on horse behavior and psychology, and spearheaded by people whose expertise is in such science, e.g., McGreevy and McLean. As one reads their writings, another body part of the horse comes to the fore - the eye.
Understanding how a horse sees (and to come extent combines vision with other senses like hearing, smell and touch) becomes the central focus of training. Learning to "see like a horse" allows one to modify riding and training method to ensure maximum trust between the horse and the human and to tackle the terrain or sporting challenges (or even management, e.g., loading on trailers) as a partnership. A horse who is on the bit, with a vertical face, cannot see to the front of it. Such a horse has to rely on the rider for guidance. If a rider is unaware of the horse's blindness in such a position, and relies on the horse's inititative and judgment, this will result in accidents like bumping into the rail of the arena, or galloping into another horse during a warm-up session at a dressage competition (all of which happens with some regularity). "Long low" position allows a horse to see in front of him better, which is sometimes what is needed. High neck carriage that is now bred into showjumpers, together with allowing for very high head carriage during the approach to the jump, allows the horse to see the jump and evaluate its height and width. Everyone is calmer that way, it's safer, and it's more efficient as a sporting strategy. The feel of relinquishing control by letting the reins get longer in front of a jump may feel terrifying, but in the end is the safer method.
The book's chapter on the eye and the following chapter on the horse cognition, are excellent and up to date summaries of current knowledge. It can be useful for designing barns and paddocks, organizing the turn-out groups and herds, backing the youngsters, and training horses for work and sport. Or just for fun, because these animals are fascinating in their own right, even if you never get to work with them. And it all arises from a good understanding of the evolution and ecology of the horse, and the history of riding, as wonderfully and vividly explained in the other chapters. Warmly recommended to all who care about horses.