On the Internet, cats rule supreme. Dogs come a close second. But we don’t talk a lot about horses. Horse zealots tend, instead, to record their devotion at length, typically in book form. And though everyone knows about Black Beauty, and the Black Stallion books, and, y’know, Seabiscuit, not everyone is aware of the healthy subgenre of spiritual books about horses. We have compiled a list of our favorite titles — none of which are, as far as we can tell, self-published items.