As I've been working on my book this week, one practice that has been pivotal with the structure is getting clear about what the core promise of the book is (non-fiction - to clarify).
What's a core promise?
The value you want the book to deliver to the reader.
It's not the topic - but what you want your book to DO for the reader. The transformation.
I'm finding that having the core promise is becoming a compass as I write, to keep me on track vs meandering off into the writing wilderness!
Let me know if this helped, and what your next step is. 😁