Dissemination of information, which can lead to people making different decisions. For example, a would-be purchaser may feel a price is a bit steep but would have bought the book anyway, but after reading the reviews decides not to in solidarity with the other reviewers.
Or an author might get feedback that their fans aren't happy with the pricisng structure, and may take their publisher to task because of it.
A market is essentially a distributed information processing mechanism. Seen in this light, information transfers that are not buying/selling can be seen as part of the market. For example, haggling has always been part of market behaviour.
Sure they do. Markets need informed buyers to function well.
You don't like the word "coercion" or "force" because you think stuffing review sites with non-good-faith-reviews it harmless, even though it contravenes the expectation that a reviewer represents a single entity who has actually attempted to consume the book. But the same logic was used during the Wikileaks Visa DDoS flood. You can choose to believe that DDoS attacks are legitimate, too. But you cannot with a straight face say that those attacks are the market at work.
I don't agree the the reviews are not "in good faith". It is wrong to write a review of a $100 Monster cable pointing out that no matter how well it works, it's a ripoff when a $10 cable is functionally identical? This is the same idea: the reviewers are expressing their opinion about whether the product is worth the asking price. Their criteria for evaluation is different from yours; that doesn't make it vandalism or coercion or a DDOS.