One way to write it is to state what the author has tried to do, compare (in your opinion) how that author succeeded, and back it up with evidence.
Here's some guidelines:
1. Write an opening statement giving essential information about the book: title, author, first copyright date, type of book, general subject matter, special features (maps, color plates, etc.), price and ISBN. (In online reviews, this can be skipped, since it is part of the blurb for the book and that data is just a few paragraphs above.)
2. State the author?s purpose in writing the book. You can often get this from their preface or first chapter. Where they don't come out and say so, you can ask yourself these questions:
a. Why did the author write on this subject rather than on some other subject?
b. From what point of view is the work written?
c. Was the author trying to give information, to explain something technical, to convince the reader of something?
d. What is the general field or genre, and how does the book fit into it?
e. Who is the intended audience?
f. What is the author's style? Did it suit your own tastes?
g. Scan the Table of Contents to see how it's organized sensibly.
g. How did the book affect you? Did you change any ideas you held because of it? How does it fit in with what you think or your own personal world view? Did it bring up old memories of yours?
h. Did the book achieve what it set out to do?
i. Would you recommend this book to others? How come?
3. Sum up the book in an elevator pitch - if you had to recommend this book to someone during an elevator ride, in the time between floors.
4. Explain how the author got his point across. What descriptions did they use? How did they tell the story - and did they keep you interested? Did their arguments make sense? Did they leave anything out or leave you unconvinced at the end?
5. Check into the author (this is fairly easy on the Internet) and see if what you find - reputation, qualifications, influences, biographical, etc. - establishes them as an authority. Do you see any relation between the author's philosophy, life experience and book you're reviewing?
6. If relevant, make note of the book's format - layout, binding, typography, etc. Are there maps, illustrations? Do they help your understanding?
7. Check the back matter. How's the index? Are the footnotes accurate and useful? What does the bibliography look like - long, short, haphazard? Make notes of what you find.
8. Summarize (briefly), analyze, and comment on the book's content and its summary. List the main topics, and briefly summarize the author's ideas about these topics, main points, and conclusions. Use specific references and quotations to support your statements. Once you have a good grip on that book, the conclusion will some simply.