Do you know yet what you're working toward, writer, with your book project?
I'd encourage you, in answering this question, to think beyond the usual categories of publishing. You may not know yet what method of publishing makes sense for you — whether you'll seek out the traditional route or publish on your own. It may be enough to know right now that you have in mind to make your book the absolute best it can be. That may be your current end goal. If so, bravo! You know your top priority and what to make your focus as you work.
Or your end goal right now may be to write the most solid first three chapters you can and then to support them with a thoughtful synopsis of the rest and to pull it all together in a book proposal. Your end goal may be that proposal right now, which means writing your three solid chapters. Hooray for that clear plan, too!
Or maybe right now, your end goal is to determine whether your idea is actually a book. It's getting clear on whether you, in fact, are writing a book. That's a terrific end goal too.
End goals change, and there are different kinds for different people and the different stages of the process. What is yours right now?