While core message and promise clarify the main idea and purpose of the book, great books also present a handful of well-developed core themes that create cohesion throughout the book and connect topics to the core message. Read this post on the importance of clarifying themes and then do the following:
1. Review the work you’ve done on message and promise and pull out the themes that will act as through-lines for the book.
2. Consider any framework you might use, any key ideas that you think will arise again and again, or any deeper philosophies that you want to hint at or rely on throughout the book.
3. Refine your list to a handful to provide direction, but know that it will likely change during outlining, research, or early writing.
4. Connect your themes to specific topics to identify opportunities to leverage the themes throughout the book.