The Mongol Art of War by Timothy May (2005) is an excellent source that I read as an undergrad. It was published in 2005, on the 800th anniversary of Temujin's ascension to the title of Genghis Khan, so there are a lot of books on the Mongols that were published that year.

It does an excellent job describing the Mongols' battle tactics, but also describing how Mongol cultural and social structures contributed to their massive success in controlling vast tracts of territory.

In terms of how they came to and maintained power over such a large portion of the globe, there are a couple of key factors. First, the Mongols lived a nomadic lifestyle and the men learned to hunt with bows and arrows from a young age. The Mongols adapted this skillset and made the bow and arrow the basis of their military strategy, so Mongol soldiers didn't need "basic training."

Second, the Mongol army was incredibly mobile. They lived off the land, rendering the need for large supply lines moot, and they were consistently able to outflank their opponents on their horses.

Third, they instituted very visible displays of terror over the peoples they conquered. May describes one vivid example of a town captured during the reign of Genghis Khan where Khan personally dragged the mayor of the town into the city square and poured molten silver directly down his throat. They Mongols also engaged in massive retaliation against anyone who slighted them. In one chapter, May describes how a military governor from the Khwarazmian Empire (which covered much of modern day Iran and Afghanistan) massacred a trading/diplomatic caravan sent by Khan to establish trade between the Mongols and the Empire. Khan retaliated by launching a massive, all-out campaign to conquer the entire Khwarazmian empire, characterized by many whole-sale massacres of civilians. Basically, people knew that if you messed with the Mongols, it wasn't going to end pretty. As a result, not many people messed with the Mongols!

Finally, the Mongols adopted one other key policy that greatly, greatly improved their ability to control vast, culturally diverse tracts of territory: religious tolerance. The Mongols did not attempt to alter local religious practices or impose any forms of religious belief in local populations in their empire. In fact, they quite readily absorbed peoples of many different religions into Genghis Khan's corps of military and political advisers. If there is one thing that will set a local population against a conqueror more surely than anything else, it's the forced imposition of religious beliefs.

Those are just some major facts, but May's book is an excellent source that explains all sorts of others. Hope that helps!