Prior to reading this, I didn't know a whole lot about Genghis Khan. I knew he conquered a huge portion of the world, and that, in Western writing at least, he and his armies tend to get a bad rap as murderous thugs. Except on that episode of South Park, where the Mongols taught everyone a valuable lesson about not letting fear lead you to try and wall yourselves off from the rest of the world.
Weatherford covers Temujin's life as best it can be determined, his upbringing, important decisions he made that shaped his life and policies as a ruler. Then he follows the Empire through Temujin's descendants, up to the Black Plague. One of the key sources is The Secret History of the Mongols, which is a group of documents that apparently detail Temujin's life, but which had been indecipherable for a long time for various reasons. First, you had to find a copy, then you had to figure out the code, which is Chinese characters representing Mongolian sounds from their language, circa the 13th century. Oh yeah, and if you were doing this in Mongolia during Soviet occupation, you'd have to be careful not to be discovered and killed. Weatherford got to be part of a team that used the deciphered text to venture into the Mongolian steppe and try to find the actual locations where key events took place.
Weatherford details the code of law that Temujin implemented for his people (and how he had a written language conceived specifically to be able to write the laws down). These laws included entirely reasonable things like no enslaving other Mongols, religious freedom for all, widows receive their husbands share of the loot from battle if he dies (good way to ensure loyalty), and that no one, including him, was above the laws. Genghis Khan comes off as generally a reasonable guy, in that his conquests are typically conducted to provide more goods for his people, but he'd just as soon get them without bloodshed (Weatherford says Temujin's overriding purpose was to preserve Mongol life). To that end, he, his subordinates, and their descendants, always give whatever location they're attacking an opportunity to surrender and plead fealty. If they do that, and keep the goods flowing, they're free to otherwise go on about their business. If they refuse - and a lot of people refuse, and commit the grave error of killing the Mongols envoys to demonstrate their refusal, because while the Mongols may have fully supported the idea of immunity for diplomats, no one else seems to have - they all die.
To be fair, when Temujin first conquered the Jurched Empire, he left their aristocrats and officials alive to oversee shipping the tribute, and those guys reneged on the deal the second Temujin left to go home. So he had reason not to offer second chances. That experience also taught him to just kill all the aristos when he took over a place, because you couldn't trust them. They'd sacrifice everyone in their city to protect what they had, so screw 'em. But maybe he's downplaying the violence, though he notes Mongols were largely opposed to torture, which at least gives them a leg up on the Catholic Church.
I don't know how fully I'd buy his argument for Genghis shaping the modern world. He does a good job of laying out his argument as he goes along, the way Temujin and his descendants encouraged the spreading of ideas across the known world, be it medicine, navigation, metallurgy, literature (Khublai Khan apparently helped spark one of the greatest eras of plays in Chinese history). How overthrowing the rulers in one area helped other peoples in those locations get up off the floor, or how resisting the Mongols helped unite feuding factions (Japan as a notable example there). It all sounds good, it's just that part of me finds it hard to attribute this to one guy and his family). It seems like too much, to the point I wonder how much of it might have happened eventually anyway. Also, Weatherford suggests the Black Plague started with fleas the Mongols picked up on an excursion into southeast Asia, and scattered from there along the various routes, and the Plague helped wreck feudalism in Europe, and I don't know how well supported that theory is, or if that isn't going too far. Still, I think he makes a persuasive argument for the considerable influence Genghis Khan had.
One interesting note is that the largely negative view of Genghis and the Mongols here in the West has only really developed in the last couple centuries. Back in the Renaissance, they were generally well-regarded in Europe, since once they stopped conquering, they were maintaining an Empire that was bringing massive amounts of goods and ideas into Europe. By the time of the Enlightenment (ha!), though, with European Imperialism such a big hit, the tide and turned, and the white man sure as hell wasn't going to acknowledge some guy from an "inferior" people as having been anything but a rapacious murderer. Certainly he wasn't someone who recognized the value of education, of the exchange of ideas, who promoted and assigned positions on the basis of skill, merit, and loyalty, rather than nepotism. Certainly not. Which is how you get all that racist crap about "Mongoloid" features being a sing of mental retardation or whatever. At the same time, this helped bolster Genghis Khan's image in the areas the Europeans were conquering, because he was proof whatever crap they were spewing about white superiority was crap. Because the Mongols had conquered more, and done it better and more wisely, than these Johnny Come-Lately Brits and Frogs (and eventually, loudmouth Americans).
'Whether in their policy of religious tolerance, devising a universal alphabet, maintaining relay stations, playing games, or printing almanacs, money, or astronomy charts, the rulers of the Mongol empire displayed a persistent universalism. Because they had no system of their own to impose upon their subjects, they were willing to adopt and combine systems from everywhere. Without deep cultural preferences in these areas, the Mongols implemented pragmatic rather than ideological solutions. They searched for what worked best; and when they found it, they spread it to other countries. They did not have to worry about whether their astronomy agreed with the precepts of the Bible, that their standards of writing followed the classical principles taught by the mandarins of China, or that Muslim imams disapproved of their printing and painting. The Mongols had the power, at least temporarily, to impose new international systems of technology, agriculture, and knowledge that superseded the predilections or prejudices of any single civilization; and in so doing, they broke the monopoly on thought exercised by local elites.'