By Louis L'Amour

Bantam Books. 357 pp. $18.95

SINCE AMERICA's premier yarn-spinner lives on a ranch where he rubs elbows with an intriguing anthropological mystery it's no surprise that the mystery becomes the subject of a book. It will be equally unsurprising if the book stretches Louis L'Amour's list of million-sales novels from 90 to 91.

The Haunted Mesa fits no category of popular fiction. It is part western, part adventure and part fantasy. The fantasy is new ground for L'Amour, but his legion of fans can rest easy. By page two, the old master has us standing with Mike Raglan, his protagonist, uneasy in the darkness of one of the emptiest places on the Colorado Plateau, ready to see our first frightening sight. Fantasy or not, L'Amour's talent for maintaining tension remains intact.

The plot concerns a question which has occupied American anthropology for three quarters of a century: What happened to the Anasazis? They appeared on the high desert about the time of Christ. They built the great cliff dwellings at Mesa Verde, Betatakin, Canyon de Chelly; the strange "great houses" of Chaco Canyon, and similar apartment-house towns across the Colorado Plateau. About 1300 A.D. these stone structures were abruptly abandoned. Why? Where did they go?

The Pueblo Indians presume these "Old Ones" were their ancestors and offer an explanation of the mystery in their origin myths. The myths tell us these ancestors left their cliff dwellings and greathouses because God told them to migrate until they found the Middle Place of the Universe. For the Hopis that Middle Place is their villages on the Hopi Mesas; for the Zunis, Zuni Village, and so forth. The Navajos, who now occupy most of what was once this Anasazi territory, have another explanation. The Anasazis had become the slaves of a supernatural figure called Never Loses. The Navajo Holy People outsmarted Never Loses in gambling games and freed the Anasazis, only to see them swept away in a whirlwind. Anthropologists waver between less romantic guesses, usually involving a terrible drought which blighted the area from 1279 to 1299, plus hostile intrusions by nomads. L'Amour, whose Colorado ranch has Anasazi ruins on three sides of it, has a better idea.

The origin stories agree these ancestor people evolved through a series of three earlier worlds -- fleeing from each after that world's society had become evil. They had emerged into this Fourth World through a "sipapu" -- a sort of birthing passage from the womb of earth. This sipapu is symbolized by holes built into kivas in Anasazi ruins, and in the similar ceremonial chambers of contemporary Pueblo religious societies. In The Haunted Mesa, L'Amour solved the mystery of what happened to the Anasazis by postulating sipapus which operate both ways -- making them gates between this Fourth World of ours and the evil Third World. L'Amour's thesis may require a little more suspension of disbelief than today's anthropological theory, but it also offers a great opportunity for L'Amour's imagination.

HIS MIKE RAGLAN is an orphan who worked the carnivals, became a magician and eventually achieved fame as the author of books debunking phony mysticism. He is drawn into the sipapu by a frantic letter from a friend -- a reclusive inventor named Hokart who is building a hideaway home on a mesa in the stony wilderness east of Navajo Mountain. Raglan goes to the mesa, where his friend has excavated a ruined Anasazi kiva. From his friend's journal Raglan learns that the kiva's sipapu is a window into another world through which sinister people freely pass. Hokart's dog has gone through the window and returned. Apparently Hokart has vanished through it and is being held a prisoner.

Regular readers of L'Amour won't be surprised that all this has transpired by page 30, about the point where most authors are still setting up the stage furniture. And it continues to move just as fast -- another good read for lovers of action-adventure.

I'll confess to having dreamed away some afternoons in places like Chaco Canyon and the cliff ruins in Canyon de Chelly, wondering about the people who built all this and then went away. Thus seeing how the imagination of a great storyteller deals with that problem was of special interest to me.

But I predict that L'Amour's trip through the sipapu into fantasy won't disappoint his fans. :: Tony Hillerman is a Southwesterner whose mystery novels -- including "The Ghostway" and "Skinwalker" -- involve Indian religions and cultures.