Ask yourself if the author presented a single idea or thesis (central argument) within her book. For example, what is a book like Harper Lee's "To Kill a Mockingbird" about? It's more than the sum of its parts--its plotlines, its setting, its characters. It's more than what happens within the story. When we walk away from the book, we are left with ideas regarding class, race, justice, gender and innocence. If we've read thoroughly and effectively, we can assume that the author has her own "take" on these issues. What do you think she wanted to say?
In most literature, writers use their talent and craft to "sugar coat" their ideas, making them easier and more pleasurable for readers to swallow. Rhetorical devices allow an author to indirectly persuade her audience not by telling readers what to think, but by showing them, by moving them to the point at which they not only think but feel the same way as the author. This can be achieved with word choice, tone of voice, humor, metaphor, point of view, irony and much more. Read your book closely. What stylistic techniques coerce you to accept the writer's view of the world? What images, characters, symbols, even words stand out to you? Why do you think they matter?
Analysis also leaves room to prove you understand the craft of writing by giving you the opportunity to tell your own audience what it was about the book you read that appealed to you. When were you impressed, enlightened, excited? When did the author convince you she was good at what she does, or when did she convince you she had the "right" idea about her subject matter?
There's also room to speak about what might have been. Seeing the gaps in a story's foundation also offers the opportunity to prove your ability to read well. For example, "To Kill a Mockingbird," while a much beloved book, is often described as being too sappy, easy and mawkish. How might Harper Lee's heavy-handed moralization deter the reader from seeing things her way? What problems can you see in the book you read that might cause the author to "lose" the reader?