Well Hillenbrand is very successful in writing a biography about a horse because of the diction and style choices that she makes. What Hillenbrand does is use Anthropomorphism.
Anthropomorphism is the attribution of human characteristics to objects which are not human. She illustrates Seabiscuit's moods in purely human terms, discussing his pride, shame, and boredom among other qualities. By treated the horses like humans, Hillenbrand creates a character that the reader can identify and care for.
I think that by diction choice, perhaps you are refering to the imagery that Hillenbrand creates. She is so good at choosing words to describe things, that when she describes locales (like Tijuana for instance) in ways that transport the reader to her destinations. Think of this sentence: "The colorful racing world that had spun itself around Tijuana withered and blew off into the sagebrush deserts." She also describes people with this vivid imagery, like Smith for example. The use of images, metaphors, and similes aids readers in deeply experiencing the story.