As I've been reviewing rather a lot of books in the past couple of years or more, and think I'm getting into the swing of it a bit, I was interested to read, via Mystery Bookshelf, itself picking up from Critical Mass blog, John Updike's book-review rules.
1. Try to understand what the author wished to do, and do not blame him/her for not achieving what he/she did not attempt.
2. Give the author enough direct quotation--at least one extended passage--of the book's prose so the review's reader can form her own impression, can get her own taste.
3. Confirm your description of the book with quotation from the book, if only phrase-long, rather than proceeding by fuzzy precis.
4. Go easy on plot summary, and do not give away the ending.
5. If the book is judged deficient, cite a successful example along the same lines, from the author's ouevre or elsewhere. Try to understand the failure. Be sure it's his and not yours?
Critical mass goes on to add a sixth, but takes a paragraph over it. It can be summed up as: "Do not accept for review a book you are predisposed to dislike, or committed by friendship to like. Do not imagine yourself a caretaker of any tradition, an enforcer of any party standards, a warrior in an ideological battle, a corrections officer of any kind."
Well, of these, I agree with (4) and the first part of (1). I don't agree with the second part of (1)* - although I wouldn't "blame" an author, I think it is fine for a reviewer to opine that a book does not succeed at what it seems to attempt. I don't agree with (2) or (3) - sometimes quotations can be apt, but I often find that reviews containing quotations are not that interesting. Selecting apt quotations is a skill, and one that is often lacking in a review, where quotations from the book can be used excessively and as a substitute for an original perspective by the reviewer. (5) is OK if there is a good example elsewhere of what an author has "failed" at, but there sometimes isn't. The additional rule (6) seems like special pleading. To my mind, if a reviewer declares any such interest openly, then it is up to the reader to read on or not. I would not agree, therefore, with (6) in a blanket sense.
OK, I realise I have just redefined a good book review as a short account that goes easy on the plot summary, doesn't give away the ending, and that understands or interprets what the author is trying to do, from the reviewer's perspective.
*Correction, thanks to Jasper in the comments to this post, I realise I misread the second part of (1). I agree with it, in fact - don't blame an author for what was not attempted.