The difference between good and great fiction and narrative nonfiction is quite simple. Good writing tells an interesting and compelling story. Great writing does this as well as tells the reader unknown truths.
I help the writers and authors I work with find the unknown truth they are trying to tell. It is the important first step that we take and one that runs throughout the work we do to continually hone down and hue to that truth.
If you can identify what it is you are trying to say as early as possible in the process, the better and clearer the writing and it will be much easier to sell.