WALKING WEST by Noelle Sickels (St. Martin's: $22.95; 308 pp.) It was a grand movement, historic in scope; an unprecedented and pregnable passage across the prairie, over the Rockies, through the desert to California and points north. As in any mass migration, it's easy to lose sight of its components, to break down the wagon trains into families, individuals; the months into days; the days into incidents. Happily, then, Alice Muller has a way with words. "Perhaps," she writes, "I can catch the simple acts of our true days and nights like a prism catches sunlight and bends it into colors you can see and name." OK, there is no Alice Muller, but there is Noelle Sickels, Alice's avatar, who has garbed her first novel in such authenticity that she may as well as have been there herself. Further, she has told the tale from a woman's point of view--easy on the derring-do, emphasis on the way the parlous journey affects the psyches of the pilgrims. The year is 1852, the itinerary from Iowa to Auburn, Calif., and the way is spattered with hazard. Seventeen set out; nine survive the trek. The others fall victim to cholera, drowning, sheer overexertion; three desert. A baby is born, another is lost. A teen-ager falls in love. A matron is molested. A taciturn spinster schoolmistress opens up, in time, "like cut flowers in a warm room." Like its characters, the story is deceptively simple and straightforward. In contrast with today's wearying razzle-dazzle, Sickels' approach is homespun, with patches of poetry. An engaging anachronism.