One of the last components of the book review is the criticisms: what’s wrong with your book? Are there sections that are unclear, or topics that should have been covered, or important questions that go unanswered, or below-average writing, or excessive detail, or unhelpful diagrams, or ….
You get the idea. The tricky bit is that this isn’t just a matter of opinion: you need to be able to back it up. WHY is this a problem? What could the author have done to make it better?