1. Try to understand what the author wishes to do, and do not blame him for not achieving what he did not attempt.
2. Give enough direct quotation — at least one extended passage — of the book's prose so the review's reader can form his own impression, can get his own taste.
3. Confirm your description of the book with quotation from the book, if only phrase-long, rather than proceeding by fuzzy précis.
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 oeuvre or elsewhere. Try to understand the failure. Sure it's his and not yours?
Update: Do also read Joseph O'Neill's wonderful appreciation in Granta: "The example of Updike is intimidating to the writer in the matter of sentences, in the matter of output, in the matter of aptitude – until, that is, one remembers that Updike himself was a stranger to intimidation, and that the Updike precedent ultimately authorizes, indeed obligates, the writer to give the task at hand his or her best shot."