Journey Into a Heart of Darkness : Tony Hillerman ventures into new territory--Vietnam--but his hero still does what a man’s gotta do : FINDING MOON, <i> By Tony Hillerman (HarperCollins: $24; 320 pp.)</i>

Tony Hillerman? In Vietnam? Say it ain’t so. The sandpaper sage of the Southwest, the author whose proudest trophy, among more than a few, is the Navajo Tribe’s Special Friend Award? Tony Hillerman bopping about the Cambodian border in a vintage armored personnel carrier a click ahead of the Cong and a 10-foot Pol from Pot?

Hey, a man has to take a breather. Mystery writer Hillerman, typically, inspires with both lungs. Besides, he reminds us, he’s an ex-grunt himself, combat infantryman, and he has a pretty good handle on that terrain too. So leave Jim Chee and Joe Leaphorn in charge of the Tribal Police (back next book) and follow Moon Mathias half a world away.

When he’s down on himself--too often--Moon, a strapping, can-do sort of man, sees himself as “a third-rate managing editor on a third-rate newspaper, sleeping with Miss Southern Rockies when she decided it was a good idea. . . .” He’s a hero to his kid brother Ricky, an oaf to his editor, a good guy to just about everybody else but a failure to his mother, steel-willed and independent, who dotes on Ricky but otherwise “isn’t a woman who needs people.”

She needs Moon now, though she doesn’t know it. Doesn’t know much of anything. She’s sedated after a heart attack at LAX, en route from her Miami home to Manila. They call Moon at his Colorado paper. She was on her way to pick up Ricky’s daughter, it turns out. Moon didn’t know that Ricky had a daughter. Mom is not a confider.

Ricky, we know, has stayed on in Southeast Asia after his discharge, setting up a helicopter business. Chopper repair for the Vietnamese army. Courier service. “ ‘Delivering things out of places where the Communists are coming in,’ explains a client. ‘Delivering property to Hong Kong and Singapore and Manila. . . . People who own valuable things will pay well for such deliveries.’ ” It’s not just an adventure, its a job. Legit. Probably.

Only Ricky has crashed and burned somewhere on the border. Died with his wife, Eleth Vinh. And his baby daughter Lila is . . . somewhere. And somebody is looking, very hard, for the cargo Ricky was carrying.

Hillerman does it well, setting us up as he sets up Moon, laying down the spoor, loping just ahead. Moon, nonplussed, gets some of the picture from a packet of papers sent to his mother by a Manila lawyer. The rest he learns from one Lum Lee, a wizened old Chinese gentleman who sifts like smoke into Moon’s airport hotel room. Ricky was flying the bones of an ancestor out of Cambodia, says Lee. The bones must be found and “ ‘placed where the fung shui is correct. Where the spirit is again comfortable.’ ” Bones, he insists. Not heroin. Sure.

Whatever, a man’s gotta do what a man’s gotta do. Something like that. Moon to Manila. Baby Lila never arrived, says the lawyer. Probably still in Saigon. which is fast coming unstuck (it’s April, 1975). Maybe Cambodia. Moon tries to trace Ricky’s friends, an elusive lot. One, George Rice, is in prison on a remote Philippines island. No problem, everybody says. Get him out of there and he’ll take you to Lila. Never mind that Southeast Asia in ’75 is a seething scrum of deserters, refugees, turncoats, fierce little men in black pajamas, cutthroat Chimeras. Hey, he’s Ricky’s bro, isn’t he?

And here is where we really begin to like Moon. Everyone assumes he’s fearless. He’s not. What he really wants to do is find the baby, most preferably in Manila, and get the hell home. Maybe even--heresy!-- not find her and get the hell home. Do his best, of course, but if she’s missing, she’s missing. Who is he, Mr. Keen, tracer of lost persons?

And here is the departure that distinguishes Hillerman’s book from semi-mindless adventure. Remember the title. It’s not “Finding Lila,” it’s “Finding Moon.” It’s discovering, belatedly and poignantly, what Moon is really made of.

Central to the quest is a marvelous dialogue, in a penitent’s box of all places. Moon, knocking about nighttime Manila, exotic and menacing, shelters from the rain in an old cathedral. He chances upon a percipient if underemployed young priest, happy to chat with a rare foreigner. In the natural course of their conversation, Moon’s confession bursts like a battlefield flare: “I killed a man. He was my best friend.” Nor is that his worst offense. That is what he did to his mother.

In due course, Moon is steering that APC through a Far East Gehenna, along with the ineffable Lum Lee, a splendid Vietnamese sailor with “Kill Communists” tattooed on his chest, and a mannequin-cool Dutchwoman whose missionary brother has decided to die among the Montagnards, thus achieving instant sainthood.

If the scenario is occasionally far-fetched, so was the war. In any case, the rousing Hillerman tale carries enough detail and insight to override any improbabilities. Hardly the Navajo Trail but well worth the detour.

“Finding Moon” is also available on four audiocassettes read by Jay D. Sanders (Harper Audio: $25; abridged).