Since the book itself is not specified, I think that you have to articulate what your impression of the particular work is. This will involve opinion and subjectivity, by the very nature of the task. Yet, there are some elements that should be present in your writing.
While you are being asked for your opinion, it sounds to me that it is important that you are able to substantiate your thoughts. It seems to me that you have to explain why you think what you think. Give your opinion and impressions, but do not think that you should not support your beliefs. For example, if you did not like the ending of the book, be prepared to give several reasons why. If you did not agree with the writer's handling of a particular topic, it would be good to offer why you felt the way you did. I think that focusing on the book's development, such as what it was about, and your impressions on how the writer handled specific elements in the book might serve as good starting points to write on this topic. It seems to me that you are simply being asked to render your opinion with some level of support as to explaining why you feel the way you do. Keeping this in mind might help with the writing of this particular task.