Message and promise communicate what your book is about and why readers should care. Read my post on how promise and message are different, consider the analysis you did on your audience (or go back to WPotW#2), and and then:
1. Describe the core message of your book in fifty words or less.
2. Describe the promise of your book in fifty words or less.
I can almost guarantee that what you first write down will not be your final message or promise (in fact, WPotW#5 and others cover that eventuality). But you have to start somewhere to give yourself direction.
WPotW is a series of posts designed to take you from idea to finished first draft in forty weeks or less. If it’s enough time to grow a human being, it should be enough time to produce a first draft. Occasionally I’ll post more than one, and you can certainly work through more than one a week to shorten your timeline. Where you go after the first draft is up to you. Hint: Second, third, fourth drafts.