A book review does not only tell you what a book is about, but also whether it achieves what it is trying to do. Therefore, a book review is more than a summary of the content (even though this is an important component), but a critical analysis of the book and your reactions to it.
While you are reading the book, take notes about the following issues:
What is the author’s main goal in writing this book? Convince you of his position on a controversy? Explain the background of an event? Raise awareness of a particular issue?
What are the author’s main points?
What kind of evidence does the author provide to make his or her points? How convincing is this evidence?
Is the book well written? Easily understandable? Good style?
What group of readers would find this book most useful (Lay people? Students? Experts in the area?)