Help understanding the sources of the book cited here

I came across the data from the book mentioned on the post, even borrowed it on the internet archive to give a quick look at it, and couldn't find the sources used for the data, so I what I'm asking is basically anything related to this data or book or both, like how accurate the data is, how they reached it, or where else I can find sources of similar kind of data. Any and every help is highly appreciated fellow comrades.