There are two ways to review a book: critically and descriptively.
A descriptive review has a lot of quotes from the book and your impressions about the book while a critical review carries your opinions about the book.
A balanced review would be a mix of both.
Before reviewing a book, it is necessary to read it carefully.
While reading, you may mark some important passages to be quoted. You may also gather your impressions about the author’s view. Most people give a summary of the book when asked to write a book review. A better way to approach this task would be to first give an introduction of the author and genre to which the book belongs( fiction, historical, biographical etc). You may then proceed to talk about the way the book is organised into chapters or subdivisions in the case of non fiction. In the case of fiction, this would be your chance to talk about the plot, theme, characters, sub plots, settings, dialogue and other elements.
One way to make your book review unique is to add your thoughts on the subject matter of the book and to comment on the way the topics have been dealth with.
You may go through www.amazon.com to see several reviews of different books.
It is important to identify the purpose of the book as envisaged by the author and give your opinions as to whether you think the author achieved his purpose.