I'd like to suggest some deeper looks at books-by-format.

1. Compare ratings for a book in print vs digital vs audio. Helps us to answer "Hey, what's a great audiobook?" if we look at ratings for the audio vs digital vs print copies, for example

2. Recommendations include a filter for format. "I want a long sci-fi audiobook; what do you suggest?"

3. In Personalized preview, account for relative ratings - something like "readers especially liked this in print version".