Pulp culture leans hard on cop books and mafia books, and now … the paperbacks have a new vein to work—the cops as the mafia. Such is the stuff of Serpico, which is slick bestseller melodrama in the true-to-life style of "the cop who defied the system," but a real guy, too, hip to things like the Village Voice and karate. Serpico … wears love-beads off duty, packs a Browning automatic, has a supermarksman rating, chops through doors and has his choice of "women who would drive readers of Playboy into a frenzy."

Despite the movie-like fanfare, Peter Maas seems finally to have one big innocent on his hands—one of the most naive or self-destructive of the "good cops," willing to finger his pals, at first in private to superiors, then to the DA, then The New York Times, then go on public record to put cops in...