IT WAS BOUND to happen sooner or later: with everyone from junk-food outlets to supermarkets discovering environmentalism, it was only a matter of time before escapist fiction got on the bandwagon. With Dead and Buried (Penguin, 224 pages, $24.95 cloth), his seventh Benny Cooperman novel, Howard Engel serves notice that even mysteries have gone green. Cooperman is hired by the widow of a truck driver who was killed in an industrial accident. Except that the widow doesn`t think it was an accident, of course. She thinks he was humped off because he knew too much about the company`s involvement in dumping PCBs into the Niagara River. After another murder or two Benny gets to the bottom of things, and this time not only does he have the satisfaction of a job well done, he can also congratulate himself - - windily -- on having done his bit to save the planet.