When students are struggling to write a paper, the reason is often that they are trying to cover too much. Paradoxically, it is easier to write a long paper about a very narrow topic than one about broader issues. Thus, as you think about Michael Lewis' book Moneyball and the science of baseball, rather than try to cover the entire book, you should focus on one single aspect of it.
Your first step in narrowing your topic might be to choose a single aspect of the game as your focus. According to Lewis, Beane used innovative methods to evaluate all aspects of the game, but focused mainly on hitting and pitching, and was less concerned with fielding ability. Rather than address both pitching and hitting, focus on just one of these. For example, you could focus on Beane's appreciation of James' "runs created" formula:
Runs Created = (Hits+Walks) x Total Bases/ (At Bats+Walks)
Next, look at one of the chapters focusing on how Beane selected players, and examine whether he really did follow his ideal and go by the numbers in selecting and trading players or whether he let other factors influence his decision. To write on this theme, you might want to go beyond the book and actually look at player statistics as supporting information in drawing your conclusions.