Historian Ralph A. Austen here tells the remarkable story of the African world that grew out of more than 1,000 years of trans-Saharan trading. During the heyday of camel caravan traffic -- from the arrival of Islam in North Africa in the eighth century to the early 20th-century building of European colonial railroads linking the Sudan with the Atlantic -- the Sahara was one of the world's great commercial highways, bringing gold, slaves, and other commodities northward and sending both manufactured goods and Mediterranean culture southward into the Sudan.  (NAF72, $19.95)