It turns out that relations between items in the TouchGraph clusters are based not on citations across texts but purely on customer purchase patterns, the data that generates the “customers who bought this item also bought” links on Amazon pages. The results, consequently, are a tad shallow. Above you see The Communist Manifesto situated in a small web of political philosophy heavyweights, an image that reveals more about Amazon’s algorithmically derived recommendations than any actual networks of discourse.
He goes on to suggest a proper book visual would incorporate citations and allow user metadata. Absolutely!