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.
Either way even if i wanted to help i could not because its asking for an opinion which for different people is different on many levels. also since the book is not specified, i can not give you any explanations of an opinion towards the book. But i will say this: that skim the book, do you like it or not? if you do/don't, then why? list several reasons behind each that you think made the book good/bad. and then expand on them. :)
The book is not listed but it is about your opinion. Everyone's opinion is different and could embarrass you if you turned our opinion in. For example people could think that dogs are the stupidest animal on earth while some think they are intelligent and cute. So I would advice you to just turn in the essay with your opinion.