Market for book reviews: Amazon version

Here's a nice description of how Amazon's reviews are ordered: at the top are the ones rated most useful, both positive and negative: The Magic Behind Amazon's 2.7 Billion Dollar Question

"Even the behavior of clicking Yes or No is elegant. Amazon tracks who rates each review as helpful, allowing each person to only vote once. This prevents "gaming the system" by voting for a friend's (or your own) review multiple times. Clicking either Yes or No pops up a quick message, saying the vote will take effect within 24 hours. (This delay also reduces gaming.)
Amazon quietly bumps the three most helpful reviews to the top. It tries to balance positive and negative reviews, so shoppers get a balanced perspective. An interesting side effect is how these selected reviews get more votes. If they are controversial (in that not everyone agrees they were helpful), their ratio goes down, allowing the most helpful reviews to bubble up past them."

And here is a discussion of how online recommendation systems might decrease the total diversity of products: Online Monoculture and the End of the Niche (HT to MR)