I think a feature where users can recommend books to others they follow/are friends with would be fantastic. It could be part of the community tab as a "Recommended to you by your community" or something like that. Maybe you could get a notification when someone recommended you a book. On that page you could see all books recommended by you with a list of users who recommended it, so you can see if multiple of your friends recommended it to you.

I don't really use the recommendations that exist now, so maybe this would be confusing, but I think it would be a really fun idea!