It should go without saying that you need to read the book thoroughly and reflect on it before you start. Once you begin writing, the key thing to remember is that this is a book review, not a book summary. Don’t spend all of your review recapitulating the content; instead, reflect upon and react to the author and book, and write clearly, concisely and critically.
You can explain briefly what the book is about in order to contextualise your review, but you should also make sure that you analyse its thesis and evidence, offer a critical assessment of its strengths and weaknesses, and appraise its value to the scholarly and/or student community. Consider matters such as emphases and viewpoints, contributions to understanding, clarity and organisation, credibility and evidence offered. Does the author present new findings or utilise new sources? How does the perspective and/or interpretation differ from the work of earlier scholars on this topic?
Mix up your commentary to avoid giving a chapter-by-chapter evaluation of the book. Even though it might seem systematic and organised to do so, this can make your review dull and pedestrian. Instead, touch on points that you find the most important and organise your review thematically.
Simply telling your readers “I enjoyed the book” or “the book is good” is not terribly useful. But telling the readers about the contributions the author makes, with specific examples, is a useful feature of any review. You can refer to the book as “interesting”, “poorly organised”, or “helpful”, etc., but in all cases use specific examples (with page references where appropriate) to support your judgement.
And finally, remain civil at all times. Sometimes (particularly if you disagree with what has been written) it can be tempting to add a flourish of wit or sarcasm to a book review, but it’s better to keep your tone professional and your criticism constructive.