This is a question I ask my clients while we're in the conception phase of their book. I ask them to list out their beliefs related to their specific book.
If they're writing a novel, this might take the shape of their convictions about the plot or the characters or the core conflict. If it's a memoir, it takes the shape of truths they've come to know because of the story they've lived. If it's a general nonfiction book, it's the ideas their book will teach.
What about you, writer? Concerning the subject of your book, what do you believe?