Donald Trump’s election suggests many things about Americans. Among them: Many of us are sick and tired of political correctness. Much of the country cringes when the president makes outrageous comments — but much of the country loves it. There is something deeply satisfying, to many people, about insulting people you’re not supposed to make fun of and questioning prevailing assumptions. The problem, of course, is that it’s often counterproductive to be offensive. Politically incorrect statements and tweets can preclude alliances and impede policymaking.

DJ Jaffe’s new book is an important reminder about the useful side of flame-throwing. Being politically incorrect …