Genghis Khan is the latest volume in the Potomac Books "Military Profiles" series, which counts some dozens of surprisingly good short biographies of notable figures in the history of statecraft and warfare.   It is a very good example of the quality of the series. 

The book provides an outline life of the Mongol conqueror, set within the context of his culture and his times. We are given a look at his origins and rise to power, are exposed to military systems of the region, and are shown the broad outlines of his many reforms, not only military but also the political and social changes that he initiated.  Of course, there's also a concise, well-written account of his many campaigns and his legacy. 

A very useful book for anyone seeking an introduction to Genghis' life and work, the history of China and Central Asia , and the conduct of war on the vast Eurasia plan.