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Thanks Valdis, I agree the maps show a great deal of consistency so it seems fair to assume the underlying data collection is consistent. Thank you so much for the link to the paper, much appreciated!
When I started doing these maps, in the early 2000s, I did speak to Andreas Weigand who was CTO at Amazon at the time. He did explain that Amazon did list books in descending order of co-sales in the their "also bought lists". He was Amazon's early data guy and helped set up all of their data analysis systems. He has since left the company. So, who knows if his original work has been changed. Just from the data patterns I see, I don't sense any major changes. If you look on Twitter or Facebook or the blogosphere... this pattern of strong red vs. blue clusters persists from a whole variety of researchers looking at a variety of social data sets. I wrote a peer-reviewed paper on this process: https://journals.openedition.org/bms/1289