Inside the Google Books Algorithm. Alexis Madrigal reports:

But what about when the company has to reach outside the web? The printed volumes represented on Google Books form a completely different kind of problem. Google’s famous algorithm can’t be deployed to search through books because they don’t link to each other in the way that webpages do. There is no perfect BookRank corollary for PageRank.

They’ve come up with something called Rich Results, which integrates more than 100 ‘signals’ – individual data categories about various book records – into your search rankings.