I find that my idea of reviewing a book has changed quite a bit from what it was some time ago.
Namely, when before it was the content which I would have commented on, probably much to the chagrin of people if they had not read the book before — for I personally much prefer to know as little as possible of any book that I am about to read. And I do not think that I am alone in that!
So, now I rather comment on the style of the writing — which is the main factor of how readable any book is. No matter how good the content, if the style is poor then the book is in effect, unreadable. Thereby, I find that offering comment on how well the book was written is the best way of portraying whether I think the book is readable.
Indeed, despite that, I would still say if the content makes sense and is reasonable. It is good to know what the content is like, even though it is a most difficult thing to determine. However, since in so many cases I can by now guess the way the book will end, I find that anything new or innovative should be mentioned.
With a finality, then, I can say that I now prefer shorter, more style-oriented overviews of books. Hopefully, also including a quote that displays a good passage from the book.