When H. L. Mencken talked, everyone listened -- like it or not. In the Roaring Twenties, he was the one critic who mattered, the champion of a generation of plain-speaking writers who redefined the American novel, and the ax-swinging scourge of the know-nothing, go-getting middle-class philistines whom he dubbed the "booboisie." Some loved him, others loathed him, but everybody read him. Now Terry Teachout takes on the man Edmund Wilson called "our...