Read the book thoroughly before anything else. Take detailed notes on your thoughts, and highlight interesting passages and words. If the book is a work of fiction, highlight different themes with different colored pens. This will help you compartmentalize your findings and make your research easier to analyze later. The more detailed your notes are, the easier your report will be to write.
Think about the book before you begin to write. Ask yourself why the author has written the book, what his rationale was for doing so. By reading about the author's life and the period of history during which he wrote, you will be able to place the book in context, and can therefore draw out subtexts with which to flavor your report.
Read books about the book and the author. You may find that your book has been written about extensively within different fields of literary criticism. If you're researching a work of fiction, there will almost certainly be a book that discusses it. Read as much as you can, and consider the extent to which you agree with the criticism you encounter. Do not be afraid to disagree.
Read reviews of the book from when it was first published. These are often some of the most helpful tools when comparing historical insight and contemporary literary criticism. If your work is a genre-defining book, how was it received? Was it panned by critics upon release? By studying the reviews in newspapers and literary magazines, you will give your book review a deep, well-considered perspective.
Write your own opinion, as long as you are prepared to justify it. Do not blindly follow the opinion of another author, and do not automatically oppose them. Discuss the aspects with which you disagree and why. University level study is about making your own conclusions from your own research. You should come to your own conclusions and not write a report rehashing other people's views.