I have to write 'my opinion' about this book. Can anyone help?

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.