Not really a secret as it moves up the best-seller lists, but Freakonomics deserves the praise it's received. At least two chapters - one on the finances of a Chicago crack gang, the other an account of the rise and fall of the Ku Klux Klan - could be books themselves.
The praise is not unanimous. Some economists have criticized Levitt's methods as imprecise or worse. Salon complains that he doesn't offer solutions to the problems he identifies. For my own part, I can't say I'm comfortable with the breathless attempts to turn him into a celebrity. And the title is...not good.
But Levitt's work is very important to social investing. Several of his studies demonstrate that cheating pays in some professions. And he has pioneered analytical techniques (mainly of question framing) that make it possible to detect cheaters in situations you might not have expected. Social investors would do well to borrow some of those tricks.