Write with purpose in mind. Edit with purpose in mind. Polish with purpose in mind. Use it as your criterion for chopping (or lack of it) and gauge your satisfaction against it. When 100% of your words are charged with meaning, your book is done.
If you don’t know what the central, golden nugget is in your book, if you haven't distilled it to that one central tenet, you'll fail to find your book's marketing message. Marketing your book becomes easier when you can zoom in on what makes you and your book special.
You can juggle fiction and nonfiction writing in your career, but you'll need organization, diligence, and the ingenuity to recognize how to give each skill set its proper time, place, and respect in your writing life.