Book reviews get both high and low reviews from readers and from authors. There is a mixed bag of opinions on the value of reviews. 

Why would a reader want to post a review for a story? There are as many reasons as there are genres of books. A few of the more dominant ones I have come across will be mentioned. I am sure there are more. Feel free to add your own in the comments.

Love the Author

A reader may have read a book that was good, great, okay, or passable as literature. What changes here is that the author is a friend, loved one, or favorite in some other way. When a fan of an author writes a review, they rave over every nuance, but they leave out the parts that another reader, a stranger to the author, may prefer to read.

A reviewer should be honest in their comments and opinions. It is a disservice to the author to fawn over them when a stranger then tears the story apart. Don't you think he or she would rather hear the dirty details from someone who cares about them?

Hate the Genre

Why would anyone even buy a book in a genre they do not like? Is it just to pad the book with low-star reviews? What is the logic in tearing apart a story in a plot or topic you don't even want to read? 

This is where common sense should prevail. If you do not like the genre, you should not try to review the story. It makes the reviewer look petty.

Didn't Finish It

Like the above reason, why would you write a review on a story you didn't even bother finishing? You don't know the way the story ended, no climax, no conclusion. 

If you don't finish the story, what is the value of your review? You are doing as the person above, padding a story with negative one-star reviews to reduce their capacity to sell.

Spoilers

For God's sake! Don't tell review-readers what happens in the story! They want to go through the journey and experience the adventure with the author. 

The Best Reviews

In my opinion, the best reviews come from people who read a book without prompt. They do not know the author, read the story with hopes for the best, but no bias or love/hate to skew their opinion.

A reader who reviews the novel of an author they do not personally know are the most honest and of the most value.

Conclusion

So, as an reader, if you want to be the best kind of reviewer, keep an open mind, leave your heart out of it, do not leave a bad review (what is the point in tearing others down?), and finally, branch outside of your fandom and read a book by an author you have no vested interest in. He or She would appreciate your honest and compassionate reviews.

Above all:  Write reviews! Authors truly love to hear from those who have read their tales.