Who is killing Cuban girls in Riverside Park on Manhattan's West Side? That's the increasingly oppressive question for crusty, middle-aging cop Bill Donovan, whose only clues--some kernels of corn, blood-draining, a burnt patch of ground, palm fronds--suggest some sort of voodoo ritual, And indeed a young local student-expert on Haitian rites turns up dead soon after cooperating with the police. But when the sister of Donovan's latest girlfriend (bartender Rosie) becomes victim #3, the chief suspects seem to be: tennis-playing Johnny Silver Shoes (perhaps the rituals aren't voodoo, but Pawnee!); or . . . a professor friend of Donovan's, a guy who's been getting kinkier and kinkier in his late-blooming, Lolita-oriented promiscuity. And finally, after some one-on-one in the dark with the killer and a backfiring decoy trap (old girlfriend Marcie, black super-cop, lends a hand), Donovan fingers the weirdly motivated psycho-culprit . . . and knocks him off in a showdown. Not very plausible, essentially familiar--but Jahn's N.Y. atmosphere and dialogue generate a gritty, authentically dank tone; and Donovan, whose sex life gets almost as much attention as the murders, is a strong, wry, downbeat presence.