Most of the time when I write a review/critique, it's of a book by an author who is dead. When I write a review of a living author, it's not my expectation that the author will ever read or care about my opinion anyway, though I'm somewhat less comfortable reviewing living authors. When it comes to writing a review of a living, less-established author, who is likely to see the review, to whom getting reviews, and especially good reviews, seems important--that's when reviewing seems more intimidating. But maybe, if you want to be fair, it's best to write the review as though the author were dead. I.e., just write what you think, don't worry about what reaction it would produce. You don't even have to worry about being right. Just think, "If I'm mistaken in my review, then that reflects on me, not on the author. We all make mistakes." It may not be easy to think that way, but it's "fair."

I earlier said, in another forum post, that when giving constructive feedback or advice, the most important thing is to be right. A review is not of the same character as a piece of advice. As a reflection of your personal relationship to the book, you don't have to see a review as something the author ought to take to heart as an actionable critique. Again, of course, we like to be right when we can, but heck, be fallible, we all are.

That all being said, well... okay, so I recently read a couple of chapters of Huckleberry Finn, and obviously Twain is long dead and his fans and anti-fans are not likely to be affected much by my opinions. I wrote a dismissive rant about Twain's mediocrity as an author and was ready to throw the book aside without finishing it. Then I finished it and found I was wrong. There was a lot that was good about it, and there were still flaws--bad parts too--but I had been unfair. So I made a point to give a fairer review and come out about how I was wrong. It didn't make any difference to anyone but me, but I didn't like having been wrong.

On the other hand, I wrote a review of a book by someone I've befriended through social media. I had already read and reviewed two of his previous books. I was one of the first to review his new book. When I read it... uh, oh, I thought it was disappointing--it was a come down from from the obvious good progress and potential that was in his earlier books. When I came to reviewing it, I didn't slam it. I didn't over-the-top praise it either. I gave what I thought a reader should expect when reading it, I reflected on the book's themes, I think I honestly depicted the book for what it was, but if it had been a book by a long-dead author whom I never knew personally, my review would have been more openly negative. So I know it's not easy to deliver a harsh condemnation if it's liable to hurt the author personally. I had been harsher on Twain, an admired "canonical" author, than on a contemporary author who had made a (hopefully) momentary stumble. So maybe I didn't reach the pinnacle of fairness. It ain't easy.

I agree with Paul on the difficulty of writing reviews when you know it will directly affect the author, either in a positive or negative way. You feel responsible for that impact. Paul is also right that reviews are not feedback for the author, so shouldn't be written as such. The aim of a review is to inform other potential readers what to expect.

In terms of rating, I would say anything less than 3 stars would have had things that serious made you think about quitting the book, or actually made you quit it. 3 stars would be an okay read overall but a mixed experience. 4 is good, and you might even recommend it. 5 stars is the rare occurrence, a book that stands apart - at least within its genre. You are likely to read it again and would be recommending it to as many people as you can.

But - you can have you own criteria. Just be consistent with it.

Good points. Reviews seem to drive so much in the book industry. I think that many reviewers are biased when it comes to urban and self/micropublished books. I've read books with fabulous,  glowing reviews that I thought were total garbage and I said this is a product of mainstream publishing? Ick! I've also read books with little fanfare that were totally awesome. I think that I'm somewhat unique with my views on art and life in general so I like to share my perspective. You will find me watching low budget films, reading authors no one has ever heard of and yes on the b-side.

For me, the criteria is pretty simple. Does the plot keep my interest, does the writing style flow, how well has the book been edited, and was I left satisfied, wanting more, or relieved to finally be finished at the end of the read. 

As a self published author myself, I have had to overcome some initial pitfalls or learning curves that may not have given the most glowing first impressions of my work. I've even had to pull some before and have them reedited, but I think most reviews hold value. As long as your review is actually based on your like or dislike of the content and not some superficial issues that may vary by the format you're reading it in, I think how you choose to review is simply a matter of taste. 

I read and review books almost weekly and I am sometimes baffled and/or amused by the things some people say in reviews or use to rate a book. Some books I wish I never wasted eye time reading, and others I'm ecstatic to have discovered a new author to follow. I know my books aren't for everyone, so I take that into account when reviewing others work also. One person's trash read, may be another's great read. Even Stephen King has critics. Just review as fairly as possible is my suggestion.

Good Luck!