When you review a book, you do that through your personal biases, after all, everybody has its own tastes, and you cannot put those aside to make an unbiased assessment. You may try it, yes, but you will never fully succeed.
So I want to state clearly what I search in books. This way you will know to put my reviews in context.
For me chess books are a source of joy (as I hope happens for you too). I have been reading chess books from more than twenty years now and one can say that I have a very classical library.
The main problem I came to realize was that I have been reading chess books 'incorrectly', only for the joy. I've read the text, but never truly stopped to think about the moves or the position. So at the end of the day, while I had a wonderful time reading those classical books, my chess level has not improved because I have not done any real work on it.
I have been trying to improve my chess for a year and a half now. In my quest to achieve this, my way of appreciating books have changed. Now I need to study seriously a book, because now I want to improve too, not only enjoy the reading. So now the attention has shifted from 'is the book well written' to 'will the book improve my chess'.
And how do you evaluate if a book improves your chess? Well, for me the easiest metric would be (elo points gained / book), but it turns out it is quite difficult to measure that, both because a book will not so dramatically improve your chess, and because the improvement is a continuous process over time, and it is quite difficult to discern the causes over long periods.
The second metric I like to put forward is (time spent thinking / book). It stands to reason that the more time you put studying chess, the higher the improvement you will experience.
When you use the (time spent thinking / book) metric to evaluate a book the first thing you notice is that exercise books are heavily favored. That's because one has to spend time thinking about the exercise to solve it, and it takes so few space in print that you can have quite a huge number of exercises. Think for example on QC puzzle book with 725 exercises, or the new book by Dvoretsky 'Recognizing Your Opponent's Resources'. Obviously it is not only about the number of exercises, as you have to find them entertaining, at your level, and they have to offer quality solutions, and I will take that into account, but overall, yes, these books are favored.
Books that try to explain some deeper knowledge, however, are heavily disfavored. They show a handful of examples and spend a lot of time on literature (or variations in the examples), but do not force you to think over the example, they require you to on your own be disciplined and don't rush through the examples to be able to read more of the literature.
I have to confess that I'm quite bad at not rushing through the examples. I do not know if it is a common thing, but I'm yet to know someone who is not afflicted by this at some level. So for this type of books the important thing is: do the author makes a good job of preventing me from rushing to the end of the book?
There are some tools at the author disposal. To find compelling examples and ask the reader to pause and think for a moment are two of those, but there is no clear and sure path. There lies the difficulty.
I may be a lone traveler on this boat, though, as I often find that other reviewers will rate books on how entertaining they are, instead of their instructional value. Or maybe rate them on its instructional value on the surface... maybe the book seems very instructional, it may provide clear rules and thorough examples, but when you try to study it you simply cannot do it and the book and its instructions will never transpire in your games.
In my reviews, I will always try to be very critique about this, I will always ponder if I have been able to really work on the book or not. Of course, this will always be a personal thing. Maybe what works for me won't work for you, but I will always (specially in bad reviews) try to explain my point of view.
So here you have my dissonant voice, I hope you enjoy it as much as I enjoy writing this pieces.