I stole this citation from John Battelle's Searchblog. But it's cool enough that I want to write about it myself.
"Rageboy" has discovered, and briefly discussed, that Amazon is now providing hypertextual citation in both directions -- showing both the books that are cited in Book A, and all the books that cite Book A. This means that you can now not only find out all about Book A, you can quickly ascertain where Book A sits in the entire pantheon of scholarship in its topical field. The similarity of this to Google's PageRank algorithm will not be lost on any of the search cognoscenti, and the potential of that alone is enormous -- in addition to reading review, finding out "people who bought also bought," and looking at best-seller lists, I can now choose my books on the basis of how often they are cited, or how broadly they cite -- powerful applications of the "hubs and authorities" model which underlies PageRank. When this is suitably combined with a nice visualization and discovery tool like TouchGraph, we might really be on to something. Three cheers for social networks of data!!