Flash Boys explains some of the mysterious behavior of Wall Street: why some firms would build a special fiber-optic line from New Jersey to Chicago to shave a few milliseconds off the time it takes to make a trade; why the stock market has frightening blips of instability, like the "flash crash" when it lost a huge chunk of value and then regained it in a matter of seconds; and what firms do with the computer-programming geniuses they hire for big money (but not nearly as big as traders' salaries).
As a piece of extended technical journalism, it's fabulous at explaining complex subjects that most people in the field don't understand. As a piece of general writing, it's a little too quick to assign white hats and black hats, especially when one of the main "black hats" (Goldman Sachs) actually gains quite a bit of gray by the end. (Don't worry, they're still a vampire squid, but they're an old and sometimes self-contradictory vampire squid. Gödel's Vampire Squid.)
The most surprising value of this is that you understand more of how computer programmers work, so if you love someone who codes and want to understand more of what they do, by the end you've have a few glimpses into the universalities of coding life -- distorted by the financial greed of the markets, which is the book's main subject, but computer programmers seem to be much the same in whatever field.
The book jumps around a little too much, trying to make a narrative out of disparate stories that don't really go together. It would have worked better as a collection of essays with a short intro and extro, I think. But it's an education in modern economics and computer science, with a good dose of old-fashioned morality thrown in. I would welcome a theologian's reflection on the nature of evil after reading this book, in fact.