Don't try to cover everything. Select an important theme or main idea and focus your writing on that area. Never stop with a simple plot summary.
First impressions are important.
Remember, the instructor apparently thought the book was important enough for a class assignment. Remember, tell enough to show that you indeed have read the book, but then get on to more important parts of the report. Take a paragraph to discuss the most significant idea presented in the book. This paragraph usually is a detailed discussion of the main theme. Point out some sections in the book that seem most helpful in understanding that theme.