For my coursework in History, my question is something along the lines of 'Why did the Mongol Empire expand so rapidly between the years 1190 to 1294.' I need two historians with differing opinions on the causes behind this, and could use some help finding them (and ideally a book written by them which expresses their opinion), would anyone be willing to help? Thanks.
As per the rules about homework in AskHistorians, we help those who help themselves! What have you read so far on the topic?
It sounds like you might be struggling with some basic research techniques.
There are a lot of great places to find books on certain topics - you can check out the wikipedia page on the Mongol Empire and look at the book list at the bottom - it will have a lot of sources to get you started. Dan Carlin's popular Hardcore History podcast (though often inaccurate) has some good books on its source pages. Once you've gotten that as a start, the best places to look for further sources is often the bibliography of books on your topic. They create a web of related readings where you can eventually find basically anything.
One thing that might give your essay a little bit of depth beyond what your teacher is expecting is to tie in some primary sources. For this any edition of DiPlano Carpini's History of the Mongols might be a great start - he was a Monk who traveled as an envoy of the Pope and attended a kurultai in the begging of the 13th century. You could also check out Secret History of the Mongols, which was a book that was supposed to teach Mongol noble children the history of their family and the Mongol Empire. Either editions of these books will also have a bibliography with good further reading.