I have come to a point where I can appreciate book reviews. For a long time, I was unsure their purpose, besides obvious reasons of summarizing a book, getting published, etc... But now I read book reviews for a few reasons:
1) To become generally oriented to a book before reading it by scholars or journals I trust.
2) To get the general 'feel' on how the book is received among my colleagues.
3) And once I am finished reading a book, I read a review to catch details or flaws in an argument I may have been short-sided to.
4) To continue the discussion. I think this is a value. Book review when done well can continue the conversation amongst scholars.
So what happens when a book review doesn't do any of these three things for me? I find it a bit frustrating and time-wasting.
What I don't read book reviews for:
1) Copious notes on every detail on each point of the book. It happens. In this case I size up the review, and if it seems like it will end up this way, I read the first and last paragraph to find the important stuff.
2) To gain knowledge on one's type-set preference or to know each typo present in a volume.
3) I also find it silly to read overly short reviews that feel more like annotated bibliographies.
What is it you look for in reviews? What is helpful to you? Why do you read reviews?