In nearly everyone's mind, for any topic which they care about at all, there is a mental model. Not, generally, a probabilistic population of models, but rather, one model: The Way This Thing Is. It may be how the economy works, it may be how politics works, it may be how romance works (or fails to). Whatever it is that a person needs to have an opinion on, because it impacts their life and they must deal with it (which means they must have a strategy for dealing with it), people have a mental model of how that thing works that they use to decide what to do.
It is always, in every case and for every person, at best, incomplete. As the saying goes, "all models are false; some models are useful."
Small wonder, then, that from time to time people come across evidence that their model is wrong. Even in fields such as war which have been analyzed from time immemorial, by people whose life depended on it, new things are discovered. For most of us, whose professions are considerably newer than that of the warrior, not only do changes upend our profession, but the old models of how things worked may not ever have been that good in the first place. It takes time for a profession to discover what works and what does not; my own current profession of software development has certainly not sorted this out yet. But when this happens, when a new mental model of How This Works comes along, there are at least a generation of people who have deep investment in the old mental model, who will be deeply resistant to any attempt to change the consensus on How This Works. This is because it would render their years (or decades) of accumulated knowledge suddenly less valuable. In many fields, this resistance is enough to prevent any change to the orthodoxy, whereas in others, there is some objective method of determining whose mental model is most correct.
In no field, not science and not warfare and certainly not economics, is this testing more rigorous than in professional sports. Every other expert in the field may disagree with you, and yet your strategy and theirs, when matched against each other in full public view, may declare yours the winner.
This book, is the story of how one paradigm was overthrown, and a new one came to replace it. If you enjoy watching the sport of baseball, it will likely add an extra layer of interest to the story for you, but I am guessing that it is by no means necessary, because this is not at its most fundamental level a story about baseball, per se. It is the story of how an intellectual orthodoxy can resist change for years (decades), and then can be shaken and overthrown by events.
Billy Beane was, we are led to believe, able to see through the common orthodoxy about what makes a great baseball player, because he was not one, and yet everyone he met in his life for years thought that he would be. While possessed of great physical talent, and obviously a keen intellect (he had been accepted to Stanford University before choosing to play baseball instead), he was unable to perform well at the professional level. Not merely in spite of this failure, but perhaps because of it, he went on to become one of the most consequential managers in the history of baseball. His fundamental insight, the foundation for all of the rest, was that Looks Are Deceiving. He turned to a succession of ever-more-nerdy sources of statistical analysis to tell him what really mattered in a player. In many ways, he was looking for players who were the antithesis of his younger self. If they were a bit chubby, or slow, or old (by the standards of professional baseball), or otherwise failed to live up to the Olympian ideal of American baseball, but they nonetheless could get the job done, then Billy Beane wanted them on his team. If they got the job done, but not in the usual way (e.g. getting a walk rather than a hit, which nonetheless got you to first base), he wanted them on his team.
The principal motivation for this unorthodoxy, was that he was manager for a team, the Oakland A's, which had far less money than its competitors. Unlike many other professional sports leagues, professional baseball teams each were free to spend as much as they wanted on player salaries. This meant that, for example, the New York Yankees could spend several times the money that the Oakland A's could, on getting the best players. It was as if they were competing in a pole vault in which different players were able to use poles of different lengths, depending on how much pole length they could afford. Because he was never going to be able to outbid the richer teams for the players which those rich teams wanted, Billy Beane was forced to find ways to get players which the rich teams didn't want, that were nonetheless just as good at winning games. This meant, that he had to have a better mental model than they did, for what it is that makes a player good.
I don't watch much baseball anymore, but as a youth I did watch many St. Louis Cardinals games with my dad, and enjoyed it. The game has a pace slow enough to encourage discussion, debate, and even prediction. Do you think he's going to try to steal a base? Do you think he's going to pitch him inside? Do you think they'll do a hit-and-run? There is enough time for those watching the game to guess what is happening, or what should happen, and enough of a pause afterwards to discuss it before the next play begins. You don't just have time to say "Yay!" or "Oh no!"; you also have time to say "Why didn't he...?" It's not as if it is a purely intellectual exercise, but more than many sports it is tailored well for intellectual analysis. For over a century now, people have been recording what happened, analyzing it, and debating it. In this sense, baseball was uniquely prepared for someone such as Billy Beane to disrupt it, by mining the data of all that history instead of relying on whether a scout liked the young fellow's physique ("we're not selling blue jeans here," he liked to say).
But in a larger sense, Moneyball is a great symbol of what has happened to our entire world, with one exception: here, it is the little guy, without huge piles of money, who has the data. Because if it had been, say, the New York Yankees who had first tried to use the power of data and analysis to crush all resistance, they could have been the Facebook, Google, or Amazon. Or, to look at it another way, this is a world where those companies were kept forever small and hungry, knocking down the walls around privilege instead of erecting new ones around their own fortresses. It can also be seen as a story of what happened when science overthrew religion in the heart of Europe, or what happened when medical knowledge started to come from double-blind trials instead of the wisdom of the ancients. There are a lot of different ways to see the quixotic quest of a small-market team in a poor city, trying to compete with the teams of larger, wealthier cities.
And it can also, of course, be read as a story about baseball. Either way, it is a great read.