According to Aristotle, there are three types of rhetorical appeals: logos, pathos, and ethos. In Seabiscuit, Laura Hillenbrand employs these three appeals to connect her reading audience to Seabiscuit and his history.
First, Hillenbrand uses logos (an appeal to logic) to develop Seabiscuit's history and explain the reasons for his not being chosen as a top race horse. Hillenbrand uses facts and logic to describe the background of the horse racing sport so that the reader understands the types of horses that are typically used for racing and the culture that surrounds horse racing as a business. This appeal to logic paints Seabiscuit as the "underdog" character which Hillenbrand will later use in the other appeals.
Next, Hillenbrand uses pathos (an appeal to emotion) to "pluck the heartstrings" of readers so that we feel a sense of pity for Seabiscuit. He is overlooked time and time again and then used simply as a tool to train other horses. Hillenbrand nearly personifies the horse by suggesting to the reader that all Seabiscuit's competitive drive was taken away from him simply because he appeared to not be a race horse.
Finally, Hillenbrand uses ethos (an appeal to ethics and credibility) to posit Seabiscuit as "an American legend"--the horse who as an underdog and rose to the top of his sport. Hillenbrand relies on an American sense of values regarding "rising to the top" to make her readers feel proud of Seabiscuit's accomplishments.
So through rhetorical appeals, Hillenbrand connects her audience to Seabiscuit.