The 1-5 rating system has a lot of issues. For books with few ratings, a few bad or good ratings can skew that number in one direction or the other. People also have very different interpretations of what each of these numbers mean.
I'd propose keeping the 1-5 rating and adding an additional question to the review page, "Do you Recommend this book?" with only a Yes or a No as an option. Maybe an optional text box to explain your selection.
On the book page, display a graph breaking this down or just show a percentage of users that recommend or do not recommend the book.
This would be very helpful in situations where a book has a 3 star rating and I'm wondering whether to read it. If most people recommend it then it may be worth my time.
Long term you could even add a feature that shows books that "similar readers recommend". Or include something on the book page that tells me if similar users recommend it or not.
I would also like to have this field. I've read a couple of books that while I enjoyed and they were good stories and I love the characters...I'm not going to go back and reread just because the plot was not my cup of tea. I highly recommend the books (and have in fact recommended them to friends who I know are likely to fall in love with the plot in addition to the characters), they just were not for me.
You could also add what kind of reader you would recommend the book to - eg. 'for fans of x author' or 'for people who like books delving into family relationships'.
Your first paragraph is exactly why we don't add so much prominence to ratings in our reviews or show them on book panes and instead have a bunch of other characteristics, like mood, pace, the character questions, etc., It's to say: the public may not like this book, but everything else fits what I typically look for in a book. Our whole ethos has been "someone's 2 star might be your next 5 star". Thanks for this idea!