Writing a review is like taking a person to a place he or she has not been to before. It is up to you to paint a picture that not only appeals but also evokes an interest.
Begin with a description of what the book is about. But make sure you do not tell the whole story and spoil it for other readers. So keep it brief. If the book is a part of a series then mention that too.
Discuss why you liked the book. What appealed to you the most. Your favourite character? Suspense? Descriptions? What emotion did the book evoke?
Make a mention of what you did not like about the book. Did you find the ending too tame or did you think the protagonist was not powerful enough?
Summarise your opinion of the book, making suggestions as to what kind of person the book would appeal to – like young adult, or those who love mystery or romance or drama and so on.
You could also grade the book on a scale of ten.
