There are none. Yep. There are no rules to govern how books should or should not be reviewed. And in turn, there are no rules on how they will be received.
Before I became a serious writer, I never thought about how my book reviews affected the author nor did I write the reviews to efficiently help other readers decide if they would be interested in reading a book. Years ago I decided only to review books I loved. My reasoning for that is I decided to be a professional writer, not a professional book reviewer. And in doing so, I don’t want something I’ve said to come back and bite me in the butt later (no matter how harmless I may see it at the time.) Maybe you don’t agree with my reasoning, which is fine, but it’s how I chose to life my life. And there are plenty of book reviewers in the world who do a great job at reviewing.
Reviewers come in all shapes and sizes. Some approach reviews methodically while other don’t put that much weight into their words. There are all types of reviews, some helpful and some not. As a reader when I’m on the fence about buying a book, I skim the reviews for intelligent pros and cons and determine if they will justify my purchase. When I see a reviewer is excessively harsh, I skip over their review. Same goes for an overly gushing review that gives no real foundation for their love for the book. It’s the in between reviews I look for to guide me.
Some reviewers gush with praise for the author and their story in a well founded manner.
Some give well thought out intelligent criticism on the shortcomings of the book without coming across negative.
Some reviewers give a middle of the road review: short, not really helpful nor hurtful.
Some reviewers give a glowing review but with only 3 stars. OR give a 1 star review before the book is even released as an ARC. Both can be baffling and frustrating for the author.
Some people review books with snarky criticism, personally attack the author or [insert whatever “unjust” review you can think of here.] Some of those people pride themselves on giving these types of reviews with others patting their backs in agreement.
The long and short of it, we can’t control how people review a book. And in turn reviewers can’t control how authors receive their reviews.
The fact is, it’s a free country, with freedom of speech, and you (authors and reviewers) have the right to express your opinion in any way you wish whether others like it or not. And I can promise you there will be someone who doesn’t like what you have to say or do. Does that mean I think you shouldn't give your opinion? No. But keep in mind, just because you have the right to freedom of speech doesn’t mean you should abuse it. Things like, condemnation, justified vengeance, shaming or imposing karma do not make this world a better place.
I think at the end of the day it comes down to this, be the best person you can be. Ask yourself if your words and actions are creating a positive force in this universe. If we all approach life in that manner, we will have so much more to gain, and our lives will be all the richer for it.