Tony Hillerman is a wonderful storyteller, which means that you believe every word he says. In "The First Eagle" (HarperCollins, $25), which like his previous police procedurals is set on the Big Reservation, straddling the Arizona-New Mexico border, a Navajo woman by the name of Old Lady Notah claims to have sighted a "skinwalker" -- a witch. In such a place, on such authority, it's easy to believe in skinwalkers. But as Joe Leaphorn, now retired as a tribal policeman, says, "Witches offer an easy explanation for unexplained illnesses." So it's just as logical to suspect, as do some biologists working for the Indian Health Service, that a new and extraordinarily virulent strain of bubonic plague is abroad.