Randall Parrish never disappoints his readers. Every story he writes is better than those that have gone before. In ''Beyond the Frontier'' he is at his best. The scene is along the trail from Quebec to For St. Louis (Starved Rock) on the Illinois River when La Salle and Tonty were blazing the way for civilization. It is a stirring story of love and frontier warfare, told in Parrish's best style.