There is no ethics specific to book reviewing. Common ethics applies. Beyond that, if you can't have a free-for-all in the literary playground, where in the hell can you have one? It's only words that are being thrown around. Most people are perfectly capable of judging such words on their own — and will do so anyway. You pick up a review, start reading it, and you realize it's a waste of your time. So you drop it and find something else to do. Perhaps some day someone will really invent a time machine, and transport Dryden, Steele, and Addison to one of these solemn conclaves just to see their reaction.