Sampling of new nonfiction; A pioneer of fact, not fiction; Westering Man, by Bil Gilbert. New York: Atheneum. 339 pp. $17.95.

Joseph Walker was a mountain man, ''half-horse, half-alligator,'' pioneers used to say. Folks declared Walker kept his mouth shut, never bragged, and dealt fairly with white man and Indian alike.

In this absorbing account of a frontier legend, Bil Gilbert, a specialist on Western lore, does more than tell the story of an explorer, trapper, and trail leader. He also draws portraits of ordinary men and women who forded streams, climbed peaks, traveled by wagon train, or lived in tepees. He sketches a 19 th-century landscape that begins in Scotch-Irish Appalachia and extends through plain and desert to the Pacific.

Western movies have glorified the gunslinger, Gilbert notes, but their type didn't build for the future. It was quiet Joseph Walker, ''and a few uncommonly good men like him,'' who really made the West.