There is a post on the above subject which appears of the Steve Laube Agency blog. It was written by Karen Ball, who was a literary agent with the Agency, but has now gone back to writing.
She said that a “good review” is helpful to readers in deciding whether the book is for them. “So here are some things, based on book reviews out there, for reviewers to keep in mind.
“A good review is balanced. It takes into account that we all have likes and dislikes, and while this book may not be our cup of tea, it could be someone else’s absolute favorite. (Hey, it could happen!) Yes, share your honest opinion. But realize that’s what it is. Your opinion. A subjective evaluation of what you’ve read. No more, no less.
“A good review is about the book, not the author. Focus on the writing, on the treatment of the topic, on the characters, on the storyline, on the research, on the facts, and so on. Don’t make judgment calls about the author’s faith, intelligence, relationships, parenting skills, parentage, or whatever. A reviewer’s job is to share your opinion of the book. You don’t have the right to go beyond that.
“A good review is about the author’s craft, not the book’s packaging. Don’t base your review on the cover or endorsements or things over which, I guarantee you, most traditionally published writers have absolutely no control. (Now, if the authors are indie, then yes, they control those things…) But remember, what you’re reviewing is the writing, not the packaging.
“A good book review doesn’t give an extensive summary of the book and then one or two lines about your thoughts. Readers can get the summary from lots of places. What they want to know is what you thought of the writing, the message, the story.
“Even more important, a good review doesn’t give away the ending/secret/mystery/twist! Please, friends, for the love of heaven, don’t ruin the read for others. If you knew who the killer was on page 2, fine, say, “I knew who the killer was by page two.” But do NOT say, “I knew by page two that the butler was the killer.” If a book has a great twist, say that. But don’t give the twist away. Have mercy on not just the readers, but on the author.
“A good book review is specific. Don’t just say you loved the book or hated it, tell us why. And tell us what specific aspect of it you loved or hated. For example:
What did you like or dislike about the writing?
What drew you to–or left you cold about–the topic or characters?
What moved or challenged or inspired or infuriated or disappointed you?”
I think this is all good advice. I would add that there are almost always some things things the writer has done well, and that praise should come before criticism. I may have a tendency to give too long a summary of the story, but when I do this, it is for two reasons: first, to show that I have actually read the book (there have been cases where a reviewer based his critique on only a sample of the book), and because I may want to make a point about the plot in my critique.