At xml.com, Erik Benson digs into All Consuming, an ad hoc web service wired together out of the Google, Amazon, weblogs.com, and blo.gs web service APIs. All Consuming trawls weblogs for book references, comes up with a top 10 mentioned books, and grabs further information from Amazon. This also includes Amazons recommendations.
Key nugget: these services require low to no permission to utilize, so developers can scratch their itch as need be.