Wisdom Lends Justice a Hand : SACRED CLOWNS; <i> By Tony Hillerman (HarperCollins: $23; 305 pp.) </i>

With “The Blessing Way” in 1970, Tony Hillerman staked an invincible claim on the Native American Southwest as a unique and fertile ground for mysteries. Hillerman’s evocations of the austere beauty of the Four Corners country where Utah, Colorado, New Mexico and Arizona come together have the force of poetry. His recordings of the place names and clans of the reservation country create a kind of litany: Coyote Wash and Standing Rock, Chivato Mesa and the Turquoise Mountain, the Bitter Water Dinee’ and the Streams Come Together People.

“The Blessing Way” introduced Joe Leaphorn of the Navajo Tribal Police, by now a lieutenant in “Sacred Clowns,” his eleventh appearance. He is a widower, just awkwardly beginning to get social again--even contemplating a trip to China with Louisa, a university anthropologist.

His young deputy once more is Jim Chee, impetuous and independent-minded as before; very smart and, like Leaphorn, a man straddling (or sometimes caught between) an ancient and demanding heritage and the clangorous modern world of walkie-talkies and sophisticated greed. Jim Chee is also distracted by love for a feisty half-Navajo, half-Scots lawyer named Janet Pete, lately returned from Washington to practice closer to home.

There is crime at hand, the brutal and seemingly unrelated murders of a volunteer shop teacher at a mission school and a sacred clown-dancer at a pueblo. The link may be a missing teen-ager, who leads the officers on a dusty chase around the reservations.

Chee is also charged with solving a hit-and-run death, and the resolution of the subplot indicates that Chee has learned, like Leaphorn before him, that wisdom must occasionally lend justice a helping hand even if it is not endorsed by the letter of the law. It is Hillerman at his most quietly affecting.

The modern world intrudes as always: Environmental activists (occasionally scorned as tree-huggers) are fighting the exploitation of reservation lands; opportunists are, as in “Thieves of Time,” fattening on the sale of Native American artifacts--the engine of the plot in the new book.

Novels inevitably reveal their authors, whether the authors intend it or not. Hillerman, who grew up among Indians and attended an Indian school in his native Oklahoma, shows both a scholarly familiarity with the myths, beliefs and practices of the Navajos and the other tribes, but as well an affectionate and unpatronizing admiration for a hard-used people.

Hillerman also owns a fine sense of humor, gentle or robust as the situation demands. Chee is desperately worried that lawyer Janet’s clan and his own may be so close as to constitute incest in Navajo belief and so preclude their marriage. He is enormously relieved to learn that the mother’s clan is the determinant in these matters, and that Janet’s clan MacDougal ancestry is not a problem. The reader is entitled to a gentle grin.

The new book is neither the most suspenseful nor the most active of the Leaphorn series. Both of the murders and the violent resolution all happen offstage. It is a Navajo police procedural, petty interagency jealousies and all. But “Sacred Clowns” is one of the warmest and most pleasing of Hillerman’s novels. His affection for his characters--and for the real world in which they live and work--has never been more appealingly demonstrated.