Marriage is the dominant theme of these eight powerful stories about men and women on the widely scattered frontiers of the Ohio River Valley and the Old West. The rawhide knot symbolizes the rough exigencies binding the lives of Conrad Richter's pioneer couples.   The title story portrays the death of Sayward Hewett Wheeler and, by long flashback, her marriage in the Ohio wilderness to a shy, bookish, outcast easterner. From "The Rawhide Knot" Richter developed his trilogy The Awakening Land. Entanglements no less curious, courtly, and consequential are seen in "As It Was in the Beginning," "Smoke over the Prairie," "The Simple Life," "The Iron Shrine," "The Dower Chest," "Early Americana," and "The Flood."   This posthumous collection shows Conrad Richter to be a prose stylist who borrowed the techniques of the ballad and cinema in compressing and fusing brilliant visual images. In a foreword, Harvena Richter notes her father's ability to create "tight mythic struc-tures." His work contains "a vastness of time and space which, however compacted on the page, expands in the reader's mind."