Consider the following type of question.
The last day in the library I saw the nice new arrival on a hot subject. I want to know about people's opinion on this book for so-and-so purposes?
The FAQ welcomes questions about math software. Then why not math books?
There might be problems with allowing book reviews, though. For example one could ask about pros/cons of Hartshorne's book. A question about such a widely used book might invite too many replies. Negative comments about particular sections might lead to some arguments.
On the other hand it will be very helpful to know the expert opinion whether some new book is interesting or not. Then again, if the book turns out to be a hit, there will be too many replies to the question.