Even though some of the books cover some pretty difficult stuff, like Nazis, abusive relationships, and drug abuse, most of them are books about sports, written by athletes. Not that athletes can't be mathletes, too…but they're not trying to go all James Joyce-style difficult on you.
The ones written by the athletes (with help, usually) are simple: they're relating their lives and their accomplishments in order to provide inspiration to the next generation of athletes. If you've ever wondered, "How do they do it?" then you'll most likely get your answer. (Hint: it's not "by watching Netflix and eating Cheetos.")
For the tougher ones, like Rome 1960 and Vengeance, the language isn't what's difficult; it's the subject matter. Politics, revenge, and death ain't light topics, so delving into those can be a bit more challenging than reading about Gabby Douglas doing a perfect floor routine.