Caldwell, Taylor.
The earth is the Lord's : a tale of the rise of Genghis Khan. (New York, N.Y. : Scribner, 1941).
http://www.fictiondb.com/author/taylor-caldwell~the-earth-is-the-lords~135479~b.htm.
Caldwell provides considerable rationale for Genghis Khan's later spectacular career of conquest. The novel draws a hugely colorful and very detailed portrait of life in Central Asia during the Middle Ages: from nomadic desert tribes, to wealthy and decadent cities like Samarkand, to the decaying empires of China.