Give a brief synopsis of the book that you read. Think of the blurb on the back cover of the book. Basically, what is the book about? There is no need to delve too deeply into plot analysis at this point; that will come later.
Mention the genre of the book and how well (or not) the contents of the book fit the genre that the author has written in. For instance, if the book is a romance novel, does the story fit the typical romance story or does it deviate from the romance plot formula?
Comment on the characters in the book. Are they believable? Does the author develop the characters fully enough? Remember, as the book commenter, you have free reign here to give your opinion of how well the author has performed his or her task.
Write about any patterns you find recurring in the story. Does the author talk about apples frequently? This may indicate an underlying pattern of good versus evil in the story (apples are characteristic of The Garden of Eden).
Give your overall opinion of the work. Would you recommend the work to others? Use personal adjectives that express how you felt while reading this book and how you feel after reading. Does the book deserve more attention or less attention than the public is giving it?