THE IMPERIAL MIDDLE: Why Americans Can't Think Straight About Class by Benjamin DeMott (William Morrow and Co.: $18.95; 256 pp.) . From its first sentence--America's "thought, character and public policy is locked in distortion and lies"--it is evident that this is an angrier book than previous works by Amherst humanities professor Benjamin DeMott, such as "Surviving the Seventies." And no wonder, we learn in a final, "acknowledgment" chapter, for it is the anger of a father defending his child. DeMott's daughter produced the critically acclaimed film "Seventeen," but when its dark portrait of working-class youth in Indiana offended its corporate sponsors, they cut her off from future funding and tried to suppress the film. Here DeMott argues by extension that our society tends to suppress any implication that class divisions are not porous. DeMott rarely documents his many indignant assertions, but he argues persuasively that by churning out such egalitarian myths as John Hughes films, where poor kids win the love of rich ones through God-given talents, our culture glosses over the hard fact that skills for success in life are more often socially taught.