Your teacher may ask for a character analysis of the main characters or that you identify the author's tone, or the book's theme and symbolism. The important thing here is to use examples from the text to support whatever you write. It is not enough to say that "Atticus is a good and fair man" to continue with our To Kill a Mockingbird example. One should write: "Several references in the text indicate that Atticus is a good and fair man. For example, when Atticus tucks in Scout after the trial, she remarks that Boo turned out to be a nice person. Atticus replies, 'Most people are, Scout, when you finally see them.'"
Actual quotes from the text are best. And always be specific. You might even identify where in the text you found the information. For example, in a book report on Jack London's White Fang, one might write the following: "In Chapter Four, paragraph three, the reader sees White Fang's determination. He hid in the bushes even though he was called by Grey Beaver and even though he "trembled with fear. . ." (Note that to avoid plagiarizing, the part taken word for word from the text is put into quotation marks.)
If you believe that a book has a certain characteristic -- exciting, boring, well-written, poorly written, well-drawn characters, etc. -- provide examples from the text to support your position.