When you begin writing your book, you don't yet know what it is. It's often more a feeling than a thing. Perhaps you can see its cover in your mind's eye, or maybe you know its name. But the style, the voice, the details and stories contained inside of it? Those are still to be discovered.
Resist the voice telling you it all has to be known right now. It doesn't.
Now is for gathering the stories. Now is for trying voice and style on for size. Now is for simply getting the words and ideas out of you.
The thrust of your book may change ten or twenty times from start to finish. You don't need to know that now. You need only lean into it.
Take new steps. Add more content. Keep the doors of possibility open, not closed.