An option to like and dislike reviews, accompanied with the possibility to order reviews based on the number of like, or like to dislike ratio.

It's my understanding that you guys are trying to stay away from any feature that would turn the app into a type of social network, but liles/dislikes to reviews could push more thought out, thorough revies to the top, so users who are undecided on where or not they should read a book could find them more easily.