On the web, that small audience can find the review easily enough

I don't think this is necessarily true -- unless you mean that small audience is looking for a review of that particular book, in which case yes, googling will turn it up. The bigger issue is that the public doesn't know to go looking for the review if they haven't already heard of the book. But I think we're in the process of developing better tools for steering people toward such things -- recommendation algorithms that work off your existing interests, etc.