Margaret Binton (Deadly Climate, 1988, etc.) is once again embroiled in a just cause, along with the cluster of friends who share the benches on the grassy triangle at Broadway and 82nd St. Here, Adrian Lavin, a young writer who was Margaret's designated buddy in a stop-smoking program, has been stabbed to death on his own doorstep, an hour after Margaret's phone call to him left her convinced he was a very frightened man. The police call it a mugging. Margaret, of course, knows better, and enlists her pals to talk to the parishioners whom Adrian was interviewing for his upcoming book on St. Martin's Church, a neighborhood landmark. This unlikely series of conversations is nonproductive, but Adrian's second, more commercial, work in hand leads Margaret into the muddy waters of child abuse and New York's overburdened courts and social services. Her search for justice ends in a cumbersome, unconvincing trap set for a murderer who now threatens her. Lots of social-injustice angst, lots of local color, but little mystery--except for the disappearance of a once charming, low-keyed heroine turned pushy and self-righteous.