What’s the optimal number of book reviews?

Virginia Postrel writes:

As an author, I want more book reviews; quantity matters more than
quality when you’re going for sheer exposure.  But as a reader, I only
want more interesting reviews, particularly of books I’m not likely to
learn about otherwise.

Newspaper book reviews, of course, are declining in number.  Here is New York Times coverage of the phenomenon.

I can think of three functions for book reviews:

1. They help people learn about good books.  If this is true, we should expect a market optimum.

2. No one much uses book reviews, but they make newspapers feel like more prestigious products.  In this case book reviews would be an inefficient form of product differentiation by making The New York Times appear more different from The New York Post than readers ideally would like.  There would be too many book reviews.

3. People use book reviews as a substitute for reading the books themselves.  I call this "book reviews as signaling."  Abolish the reviews and either a) people will have to go read the books (an even more wasteful form of signalling), or b) people will forget about literary matters altogether, which lowers signalling costs.

I use book reviews as I would use ads for books and blurbs for books.  I just want the bottom line.  I would be happier if newspapers published many more one-paragraph book reviews, but with very clear and definite evaluations.  Entertainment Weekly does just this, although I find their taste in books unreliable.  Nonetheless I am not alone in my preference, and I believe that few people read long book reviews.  That makes me think there is something to #2.