A solid cop novel form a former N.Y.C. policeman: first- novelist Grant spaces some chilly murders with gutsy shoptalk and some scattershot sleuthing in between as a few men from the NYPD begin to suspect that one of their own is a monster. Two men, identifying themselves as cops, enter the stale apartment of a major Harlem drug-dealer who's never afraid of cops''predictable as the sunrise...He died believing that.'' Before long, detective veteran Brian Shannon and Alex Rose from Internal Affairs (sent, to Shannon's anger and disgust, to partner him) will track through a nightmare to an unthinkable target: a bad cop. There are more neat murdersa crooked lawyer and an art dealerand will there be a fourth? Yes, but not the one plannedthis one's of a state senator on the mayor's tail for laxity in pushing drug-busts. As Shannon and Rose guess, worry computer tallies, interview, check out and stake out, political and hierarchical vibrations begin in the Department and City Hall. In the center is a shiny, polished villainthe seemingly invincible, much-honored head of an elite narcotics unit. Shannon and Rose work in harness (there's the obligatory booze/bonding scene and confessions about lousing up their personal lives for the work they love), and even before the final deadly chase, it all adds up. But with the hot potato bouncing up the steps of City Hall, who pays the tab? Grant has a quick eye for catchy detail (police-shy apartment dwellers close their doors like ``fiddler crabs disappearing into sand holes''); the action is tough and efficient; and the talk and ambiance have a sterling authenticity. A debut that weighs in with promise.