I have an interest in Inner Asia, and so have read through a good chunk of the recent stuff on the region. Recently began reading Empires of the Steppe, a book written in 1939. My professor inquired and I said I was reading it for the literary value due to how important it was when it was written. He then told me that a lot could be learned by reading an older work if I was familiar with the more recent treatments. Can someone please explain what he means by this?
Our views of history will naturally change over time. Different cultures and timeframes will naturally treat historical periods and regions differently. Essentially, he's saying that if you're familiar with how more recent historians treated the nomadic empires in Central Asia, it can be good to get a grasp on how our views on them have evolved given the way Grousset treated them in his book in juxtaposition to the way historians who have published more recently do.
I think the best definition of historiography is that it's the study of history writing.
Especially if you look into the source materials for both timeframes, you can see how our views have shifted (if at all) given what newer information was available to the more recent historians.
Books like this can also help explain sentiments toward various peoples and places and ideas at different times and that's an incredibly valuable resource to an historian.
Those would be my general thoughts on what he told you. If you have any more specific questions, I'd be happy to take a crack at them as well.