Neither rain, nor sleet, nor gloom of night will keep the postman from completing his rounds -- even if it is Anthony and his team of six mules on the celebrated and very steep trail into the Grand Canyon from the South Rim to Supai. Brown's marvelous introduction to the canyon and its lore for youngsters dramatizes the historic mail route in large watercolor illustrations and simple, direct text.  (SWU468, $7.95)