The Fallen Man by Tony Hillerman

Sprawled on a ledge under the peak of Ship Rock mountain for eleven years lies an unknown body, now only bones. At Cabyon de Chelly, three hundred miles across the Navajo reservation, a sniper shoots an old canyon guide who has always walked that pollen path in peace. At his home in Window Rock, Joe Leaphorn, newly retired from the Navajo Tribal Police, connects skeleton and sniper, and remembers an old puzzle he could never solve.