Genghis Khan and the Making of the Modern World by Jack Weatherford

While this overextends its arguments, it's still fun to read because a) it does convey a sense of how wily Genghis was, and b) Conan the Barbarian jokes.

Chroniclers of the era attribute to Genghis Khan the highly unlikely statement that the greatest joy a man can know is to conquer his enemies and drive them before him. To ride their horses and take away their possessions. To see the faces of those who were dear to them bedewed with tears, and to clasp their wives and daughters in his arms.

On a practical note, one has to fill in the details with google maps and wikipedia to grok the trade-offs Genghis faced. ie. for any given history, what were the options and consequences? However, since Genghis' beginnings are historically obscure, it's easier to start with a history of Subutai to see how he develops and use that as a proxy for early Genghis.

Empire expansion was based on carrot-and-stick offers. Since the governance of any given city back then was a mixed bag, the offer of low taxes and guaranteed safe inter-city commerce vs annihilation seems to have persuaded many cities to opt for the strange new lower tax regime, ie. throw the local misruler under the Mongol bus.

With his penchant for finding a use for everything he encountered, he devised a powerful way to exploit the high literacy rate of the Muslim people, and turned his unsuspecting enemies into a potent weapon for shaping public opinion. Terror, he realized, was best spread not by the acts of warriors, but by the pens of scribes and scholars.
What would the Mongol Horde look like on Twitter?

Empire management was a mix of top-down know-how appropriated from the Chinese states, which came with its own baggage, ie. inter-domain cooperation problems.

Genghis Khan would be more accurately described as a destroyer of cities than a slayer of people, because he often razed entire cities for strategic reasons in addition to revenge or to provoke fear. In a massive and highly successful effort to reshape the flow of trade across Eurasia, he destroyed cities on the less-important or more inaccessible routes to funnel commerce into more routes that his army could more easily supervise and control.
Mongol BRAC

Too late in his life did Genghis realize that his empire needed shared myths and ideals, in addition to shared directives and laws.

You may conquer an army with superior tactics and men, but you can conquer a nation only by conquering the hearts of the people.

He tried to promulgate his own religion, which failed likely because a) it was built top-down instead of from the grassroots, b) his disregard for "rules of engagement" seems to have been inoperant here which prevented him from using the tactics of other religions (judicious executions and the cooption of the ruling class), and c) nobody other than Genghis had a vested interest in seeing it succeed.

In twenty-five years, the Mongol army subjugated more lands and people than the Romans had conquered in four hundred years. Genghis Khan, together with his sons and grandsons, conquered the most densely populated civilizations of the thirteenth century. Whether measured by the total number of people defeated, the sum of the countries annexed, or by the total area occupied, Genghis Khan conquered more than twice as much as any other man in history.
But Purev thought nothing in the history should be taken at its literal meaning. According to him, Genghis Khan was the most powerful shaman in history, and the text was a manuscript of mysteries that chronicled, in symbolic ways, his rise to that position. If it could be unlocked, it would again provide a shamans blueprint for conquering and controlling the world
Of her four subsequent children born after Temujin, Hoeluns youngest son bore the name Temuge, and the youngest child and only daughter was named Temulun. All three names seem to have the common root of the verb temulwhich occurred in several Mongol words meaning to rush headlong, to be inspired, to have a creative thought, and even to take a flight of fancy. As one Mongolian student explained to me, the word was best exemplified by the look in the eye of a horse that is racing where it wants to go, no matter what the rider wants.
Such stories offered assurance that the old khan was truly dead, and at the same time they heaped shame and opprobrium on the court of the Naiman, the next target of Temujins campaign. Propaganda and control of public opinion were quickly emerging as Temujins primary weapons of choice.
The fighting among his sons made him keenly aware of how much work he needed to do to preserve the empire after his death. His sons did not match up to the needs of the empire. While pursuing his great quest to unite the steppe tribes and conquer every threat around him, he had never devoted the attention he should have to his sons, and now they were all reaching middle age and were still unproven men.
He tried to teach them that the first key to leadership was self-control, particularly the mastery of pride, which was something more difficult, he explained, to subdue than a wild lion, and anger, which was more difficult to defeat than the greatest wrestler
The discussion continued for some time, and according to Rubrucks own account, it was obvious that he did not fare well in the sometimes acrimonious arguments. He was unaccustomed to debating with people who did not share his basic assumptions of Catholic Christianity. Evidently, Mongke Khan recognized the problems he was having and suggested that all the scholars present take time to write out their thoughts more clearly and then return for a fuller discussion and debate of the issues.

The Mongols loved competitions of all sorts, and they organized debates among rival religions the same way they organized wrestling matches. It began on a specific date with a panel of judges to oversee it. In this case Mongke Khan ordered them to debate before three judges: a Christian, a Muslim, and a Buddhist. A large audience assembled to watch the affair, which began with great seriousness and formality. An official lay down the strict rules by which Mongke wanted the debate to proceed: on pain of death no one shall dare to speak words of contention.

Rubruck and the other Christians joined together in one team with the Muslims in an effort to refute the Buddhist doctrines. As these men gathered together in all their robes and regalia in the tents on the dusty plains of Mongolia, they were doing something that no other set of scholars or theologians had ever done in history. It is doubtful that representatives of so many types of Christianity had come to a single meeting, and certainly they had not debated, as equals, with representatives of the various Muslim and Buddhist faiths. The religious scholars had to compete on the basis of their beliefs and ideas, using no weapons or the authority of any ruler or army behind them. They could use only words and logic to test the ability of their ideas to persuade.

In the initial round, Rubruck faced a Buddhist from North China who began by asking how the world was made and what happened to the soul after death. Rubruck countered that the Buddhist monk was asking the wrong questions; the first issue should be about God from whom all things flow. The umpires awarded the first points to Rubruck.

Their debate ranged back and forth over the topics of evil versus good, Gods nature, what happens to the souls of animals, the existence of reincarnation, and whether God had created evil. As they debated, the clerics formed shifting coalitions among the various religions according to the topic. Between each round of wrestling, Mongol athletes would drink fermented mares milk; in keeping with that tradition, after each round of the debate, the learned men paused to drink deeply in preparation for the next match.

No side seemed to convince the other of anything. Finally, as the effects of the alcohol became stronger, the Christians gave up trying to persuade anyone with logical arguments, and resorted to singing. The Muslims, who did not sing, responded by loudly reciting the Koran in an effort to drown out the Christians, and the Buddhists retreated into silent meditation. At the end of the debate, unable to convert or kill one another, they concluded the way most Mongol celebrations concluded, with everyone simply too drunk to continue

In his short and disastrous reign, Guyuk had purchased vast amounts of goods and paid for them with paper drafts on the promise that the paper could be converted into gold or silver by the merchant when needed. With Guyuk dead, many local officials and advisers no longer wanted to pay off these bills issued by the late khan. Mongke, however, astutely recognized that if he did not meet the financial obligations of Guyuk, it would make merchants and other foreigners reluctant to continue business with the Mongols. Mongke Khans decision to pay those debts prompted Juvaini to ask, And from what book of history has it been read or heard . . . that a king paid the debt of another king?
The Mongols allowed each nation under its control to continue minting coins in the denominations and weight they had traditionally used, but they established a universal measure based on the sukhe, a silver ingot divided into five hundred parts, to which each of the local currencies was tied. This standardization of varied currencies relative to the sukhe eased problems in accounting and currency exchange for both merchants and government administrators. Thus, standardization of currency allowed Mongke Khan to monetize taxes, rather then accepting payment in local goods. In turn, the monetization allowed for standardized budgeting procedures for his imperial administration, since instead of accepting taxes in goods, the Mongols increasingly accepted them in money. Rather than relying on government officials to collect and reallocate tribute of grain, arrows, silk, fur, oil, and other commodities, the government increasingly moved money rather than goods. For the first time, a standardized unit of account could be used from China to Persia. So long as the Mongols maintained control of money, they could let merchants assume responsibility for the movement of goods without any loss of government power.