There were many points in that book, but one of them might be that looking at historical data on income can be a bit easier for data gathering if the civilizati

There were many points in that book, but one of them might be that looking at historical data on income can be a bit easier for data gathering if the civilization in place at the time requires filing of taxes. If those taxes are on income, you get a much better set of visibility for data points than trying to estimate wealth. (Usually it seems taxes are on income...) One of the amazing things about the book, was separate from ones given beliefs about his recommendations regarding inequality, was that Picketty had painstakingly amassed all this data on income and wealth across a huge range of history, across multiple eras, from before the victorian age in many cases. That alone is a great accomplishment - and lets one start to quantitatively analyze fundamental economic questions of the modern day.