“The air is full of ideas. They are knocking you in the head all the time. You only have to know what you want, then forget it, and go about your business. Suddenly, the idea will come through. It was there all the time.” Henry Ford
“You have to have an idea of what you are going to do, but it should be a vague idea.” Pablo Picasso
Usually, the concept for books comes from seeing the world around, and from imagining situations that fit the situation. Always think about the 'why' behind conditions and attitudes.
A vague idea provides choices. Then, it grows. From the get go, don't presume to know what you're going to write about. Write only what comes to mind. Connect the dots when writing as you think. When you review and edit your version, the stuff you disagree with or that don't make sense can be deleted.