And here is an interesting take on Valdis' analysis:

Wow, that's pretty interesting. Validis's writeup explains this structure as a reflection of political tendencies in the readers. However, I can think of at least one other generative mechanism. If Amazon's "buddy book" marketing is effective (and I'm pretty sure it is, because it regularly "gets" me), then the system tends to reproduce its own links. If there was any polarization in preferences when the system was launched, then new buyers would have been steered toward these same choices (and given limited budgets, therefore away from others). You could spin this out into friendship networks and back too: Discussing books with like-minded friends leads to recommendations for other books from a similar political perspective, which are then purchased and contribute to Amazon's data. Amazon's buddy book system may actually cause political polarization, and would appear to at least represent a source of intertia in that phenomenon.