by Tony Hillerman, Harper & Row, $15.95. Lieutenant Joe Leaphorn and Officer Jim Chee, of the tribal police, are so observant of the slow niceties of traditional Navajo etiquette that it seems unlikely that they can ever catch up with a murderer. They do, of course, and their cautious pursuit gives Mr. Hillerman time to offer his audience an affectionate study of Navajo life and of the strange, tough, beautiful country that the tribe inhabits.