THE BLUE NILE (308 pp.)Alan MooreheadHarper & Row ($5.95).
As Shakespeare well knew, the sun's heat bred serpents and other monsters out of the mud of the Nile. With The Blue Nile, this ancient river of mystery has now been made the object of two studies that employ all the modern arts of research to dispel myths and muddy misconceptions. Alan Moorehead, an extraordinary journalist-turned-historian who examined the history of one of the river's sources in The White Nile, tells in his latest book what succeeds the great civilizationsEgyptian and Greekthat rose and fell with the Blue Nile as its annual...