The instant grab of violence which McBain and his Steve Carella dispense, and dispose of quickly, vitalizes this story of two teenage cousins--one of whom, Patricia, turns up with bloodied palmprints and a torn dress while Muriel is found slashed to death in the streets. Then there is Andrew, Patricia's brother who loved Muriel too soon, too much. . . . Easy to type as ""O"" for operant procedural in a series which has every reason to retain an ongoing readership.