As a spiritual director, discernment is one of my favorite things.
What is the invitation? How are you being invited to move forward right now? How do we know this? What are the indications? In spiritual direction, these questions get tended a lot.
They get tended in book-writing, too.
First you discern the invitation to write the book. Then you discern the "how" of writing that fits your actual life. Later, you discern your publishing team and your personal path to publication. Later still, you discern a marketing and publicity plan that aligns with who you are and where your work is leading you.
Discernment, discernment, discernment. It happens regularly in this process. Or, at least, the wise path to publication allows this to be so.