Meet Rachel Alexander, dog trainer turned shamus, and her pit bull Dashiell. Rachel gets $500 a day plus expenses; Dash gets $50, but (a nice touch) Rachel covers his expenses. Rachel won't work without Dash, but that makes sense when the case is a homicide witnessed by a dog--Magritte, the missing basenji who belonged to hit-and-run victim Clifford Cole, a painter whose body was found out on the Christopher Street pier. Gay-bashing, say the unmotivated cops. But Cliff's friend and neighbor Dennis Keaton smells premeditation; why else would Cliff have gone out to the lonely pier in the dead of night? It doesn't take Rachel long to find Magritte, or much longer to track down another, human, witness (in an unexpectedly affecting scene) who tells her the dog was nowhere near the pier where Cliff was killed. But just when Rachel's ready to pounce on the likeliest suspect, he drops dead, victim of an inventive poisoning, in the middle of the Westminster Dog Show--and then, lest she be lulled into assuming the killer's gotten his just deserts, Rachel realizes she's being stalked by the real, unpoisoned killer, whose motive is as surprising as it is logical and chilling. Benjamin, a veteran of children's books and dog-training guides, has spun a first mystery that's de rigueur for dog fanciers and highly satisfactory even for readers who prefer two- legged animals.