Valdis Krebs has extended his analysis of book buying habits, originally written a few years ago: The Social Life of Books: Visualizing Communities of Interest via Purchase Patterns on the WWW
One of the cardinal rules of human networks is "Birds of a feather flock together". Friends of friends become friends, and coworkers of coworkers become colleagues. Dense clusters of connections emerge throughout the social space. The usual pattern found throughout social structures[and many other complex systems] is dense intra-connectivity within clusters with sparse inter-connectivity between clusters.
His new analysis, Political Patterns on the WWW, looks at New York Times Bestsellers List and created an impressive map that shows people who buy "liberal" books tend to only buy other liberal books. And those who buy "conservative" tend to only buy conservative. There is very little overlap between the audiences.
Here is the network map. Have a look at his writeup directly for more info (and a bigger image).