Lords of the Bow by Conn Iggulden

Second Hand 

The gathering of the tribes of the Mongols has been a long time in coming but finally, triumphantly, Temujin of the Wolves, Genghis Khan, is given the full accolade of the overall leader and their oaths. Now he can begin to meld all the previously warring people into one army, one nation. But the task Genghis has set himself and them is formidable. He is determined to travel to the land of the long–time enemy, the Chin and attack them there. The distances and terrain–the wide deserts, the impenetrable mountains–make it a difficult venture even for the legendary Mongolian speed of movement, but the greatest problem is that of the complex fortifications, a way of fighting wars of a settled urban population which the nomadic Mongolians had never come across. Finding ways to tackle that and keeping his tribes together in a strange environment presents another new and exciting challenge for Genghis Khan.