What we write when we write reviews

By way of reviewing the worst book review(er) ever, Robert Pinsky says that when he first started writing reviews, one newspaper gave him the following set of guidelines:

1. The review must tell what the book is about.
2. The review must tell what the book's author says about that thing the book is about.
3. The review must tell what the reviewer thinks about what the book's author says about that thing the book is about.

Pinsky then goes on to point out that hardly any reviews these days do all three things. At most they do one or two of them. Also, those who prefer Rule 1 tend to avoid Rule 3, and vice versa.

Which raises, for me, larger questions: What are book reviews for, anyway? Are they a "Consumer Reports"-style report, as Pinsky puts it? (To buy or not to buy?) Are they criticism, and if so, what does that mean?

I like to read book reviews and have written a few in my day...but I honestly don't know the answers to these questions.