The Blessing Way by Tony Hillerman

Alternate cover edition can be found here.

Homicide is always an abomination, but there is something exceptionally disturbing about the victim discovered in a high lonely place, a corpse with a mouth full of sand, abandoned at a crime scene seemingly devoid of tracks or useful clues. Though it goes against his better judgment, Navajo Tribal Police Lieutenant Joe Leaphorn cannot help but suspect the hand of a supernatural killer. There is palpable evil in the air, and Leaphorn's pursuit of a Wolf-Witch is leading him where even the bravest men fear, on a chilling trail that winds perilously between mysticism and murder.