I only know the very basics of the mongolians, it all rather fascinates me on how quickly they expand and then collapsed, killing so many millions of people in the time. Any books on this period that are good?
First, check the book list. It is linked in the sidebar. Check in the Asia section. There is a special list for Mongolia.
In addition, I'd recommend:
The Mongols and Global History by Morris Rossabi, 2011
The History of Mongol Conquests by J. J. Saunders, 1971
The Mongols by David Morgan, 2007.
If you are wanting an entertaining read about Genghis Khan and the Mongols, from the book list Genghis Khan and the Making of the Modern World by Jack Weatherford is the book for you. This recommendation is to be taken with a grain of salt. Weatherford isn't exactly a historian and his book is really considered popular history. I'll readily admit I've read it and I've used it for a source on this sub before, but you have to remember it's really entertainment. The other books I listed above have more accurate information than Weatherford. I'd say read one of those if you want accuracy over entertainment.
EDIT: I'm actually going to let /u/bigbluepanda take this one instead of writing my own response.
Hi, just going to add in my own response seeing that /u/krishaperkins hasn't come back just yet. I've written a bit on the general motivation behind their expansion as well as what made their expansion so destructive - here and here, so I'll touch on those as well.
The inherent nature of the Mongols
The Mongol Empire under Genghis was the unification of various nomadic tribes across the Eurasian steppe - being tribes, they were more accustomed to raiding and looting towns rather than conquering and consolidating their empire. Looking at this image of the general area you can see that the Mongols would be able to quite easily find wealthier societies near them that were worth the effort.
As we know, nomads of the Eurasian steppe were mainly pastoralists, herding their livestock to new grazing regions whilst also bringing their entire living area. Being pastoralists, they could not generate the goods that wealthier societies could produce (China: silk, tea, china/porcelain, as well as many other products that civilisations across Asia produced) and so in turn had nothing to trade for these goods.
Reasons to expand
Because of them:
being unable to create a fully self-sustainable source of income
being unable to enjoy luxurious goods
whilst being being bordered by significantly wealthier societies across Asia that were also close to the steppe, the Mongols were pretty much in the best position to come into all of them. This was the initial drive for the Mongols to expand, as further evidenced in how they established the Silk Road under a single political entity.
However, invading other areas however could then result in two things - either the society you're invading has allies that will fight you too, or not - which as a result stimulated further conquests. This doesn't apply to all the regions surrounding the Mongols, as most obviously seen in how the Mongols managed to conquer China.
Within China
China was fragmented into three different dynasties, the Song, Jin, and Xia, and so due to the political instability and the unwillingness to help each other, the Mongols managed to establish the Yuan dynasty of China. The political fragmentation of China was the key in the Mongol consolidation of east Asia, as it allowed them to have access to technology and wealth sources which in turn catalysed their expeditions elsewhere.
/u/krishaperkins has already given you some great books, but I'll just add some of my opinions on some.
The Mongol Conquests in World History - May, T.
A good book that covers the chronological history of the Mongol Empire starting with Genghis' unification to the retreat of the Mongols to their eventual retreat into Mongolia. It also talks about the short and long-term impacts of their rule as well as several other outcomes due to their administration efforts. Like most modern texts dealing with the Mongols, it views them through a much more objective lens (as opposed to earlier sources that still very much referred to them as barbarians). Readable as well.
In regards to the Weatherford book, it's a rather refreshing breath of literature in that it dealt with his life in a much less downgrading fashion by attempting to paint him in a positive or at the very least neutral light. A piece of revisionist history, sometimes hyperbolic in fashion, as well as glossing over a few facts that probably weren't true, most likely because they didn't fit the image of the Khan being a civilised, peace-bringing lawkeeper that the book suggests.
