Genghis Khan by John Man is very good, it focuses on Ghengis and also the mongols as a people, in addition to an epilogue about his legacy in the east, and how he is venerated in Mongolia, and, more interestingly, Chinese folklore. He comes across as a ruthless warlord, but also a thoughtful and pragmatic ruler.
However, it doesn't spend enough time on his successors and the rapid decline of his empire. There's also a slightly tiring personal "spiritual quest" to find the Great Khan's grave at the end.
Mostly however it's a very good, detailed and accurate account of a figure that remains quite mysterious and demonised in the West.