A number of university and college writing centers have published extensive guides on writing book reviews. These guides typically include: a description of the book review format, a step-by-step guide to constructing your review, and questions to consider or address when writing your review.
Recommendation: Choose one of these guides to help you structure your review. If you get stuck or need more ideas, check out one of the others - it will likely offer some questions or points to consider that weren't included in the first.