What makes a good, but fair, review?

My emotional response when reading equates to how many stars lost or gained?

​One. One star for each criterion.

​"But I loved that book! It changed my life!"
"I absolutely hated it! The main character was an absolute pain. The number of stars kept or removed should equal the number of emotions I felt!"
Consider this: Does your emotional response to a book or a character mean that you don't take the rest of the book into account? 

"Okay, but what if I didn't like the book at all but feel bad about removing stars? I can just remove one star and that'll show the author I didn't love it. Right?"
Look at it this way: Removing one star because you feel bad for the author actually shows everyone else, and the author, that you still enjoyed the book, not how you really felt. 

"But they worked so hard writing their book and I don't want to discourage them."
I'll grant you that, but let's acknowledge these as well:

• Are you helping the author provide their best written work if you are not being honest with them about their book in your review?
• How are you helping future readers find good/great/excellent books by providing reviews that don't truly express how you felt about the book? 
• If you are providing constructive criticism, nicely, then a true author will consider the error --- if there was one....readers make mistakes too! --- and fix it. When they have the money to do so. It costs money every time they have to republish, even to fix those small mistakes, so it may take a while.
                     

​"I DNF'd this utterly boring, horrible book but I'm still leaving a review."
Mull this over for a sec, will ya?

• How could you possibly know if the author's book got better or worse to be able to leave a fair and thorough review?
• Even if you write in your review that you did not finish the book, what star rating should that equate to?
  
• If you decide a book isn't worth your time to finish reading it, then why it is worth your time to rate and/or review it?
  

Now, some places like Goodreads may allow you to leave a review without a star rating. In this case, explain why you did not finish the book in your review without the star rating. That way it doesn't affect the author's standing and you get to share with future readers why you put the book aside, threw the book away or chucked it across the room unfinished. Win-win.

Writing a review for fiction books can be as simple as one star per criterion. 

1. Are the characters believable?
2. Does the plot make sense or have holes? Does it plod along or feel rushed?
3. Is the dialogue realistic?
4. Did you stumble when reading because the writing mechanics, grammar or spelling threw off the flow or made you re-read a section?
5. Did the author make you angry, happy or cry?

That's it! ​

If you felt nothing when reading the book, but the characters were well written, there were no plot issues that you saw, the dialogue and writing mechanics were good, then it was simply a 4-star read for you. The next reader may have felt that connection with the characters to make it 5 stars, while another found some grammatical mistakes and a plot hole others have missed, but loved the characters - that would technically be a 3-star book.

If a book you read wrecked you emotionally, one measly star doesn't seem like enough to show how much it affected you, but that's what the written part is for! Share your emotional upheaval caused by this author's book in the review and provide examples. Your feelings will shine through!

Non-Fiction book reviews have slightly different criterion but a fair review is still needed. 

1. Did you stumble when reading because the writing mechanics, grammar or spelling threw off the flow or made you re-read a section?
2. Does the author cite where they got their information or have first-hand knowledge of the material?
   
3. Is the information relevant or is it out-of-date and could people find it helpful?  
   
4. Does the author write like they are an expert in their field or is it childish and unprofessional?
   
5. Did you have an emotional response?
   

If the book is not intended to create an emotional response, e.g. a programming book, then you can simply split the fifth criteria star between criteria three and four or add the remaining star to either criteria three or four. Either way, it still makes 5 stars. 

If you can give a fair assessment of a book, minimizing your emotional bias, then future readers will know the book's true rating and be able to decide for themselves whether to try it or not. Your emotions are what makes your reading experience unique and your reviews subjective, but that doesn't mean everything else should be ignored.