Whether you publish your book traditionally or publish it yourself, the best thing you can do for your writing career at this point is to hire an editor, copyeditor, and proofreader. But what's the difference between them?
What does the editor do?
The editor will help you mold your book after you've written it. Here are two editorial comments I've received that stick in my mind. For my YA novel, Dancing With the Devil the editor suggested I combine two characters into one. I mean, did I REALLY need two best friends? Nope. I rewrote the manuscript and combined the two, and the story was much stronger for it.
The second comment, in the form of a question was the most valuable. Steve Geck was my editor at HarperCollins, the place that published my middle grade, The Curse of Addy McMahon. It begins with a fight between two besties, right before the winter break. New Year's comes and goes, there is a blizzard that delays school reopening, and finally they go back to school. At the top of the manuscript he scribbled: She doesn't have to celebrate Christmas, but do you think it's odd it's never mentioned?
That one question spurred a much deeper revision. It led to the creation of a sad, still-wrapped gift that sat on Addy's shelf for a large part of the book, reminding her––admonishing her––to own her mistake. It gave me a way to show aspects of both girls' characters, and it led to a much deeper level of heart for the entire book.
What does the copyeditor do?
When you're ready, the copyeditor checks for errors in spelling, grammar, punctuation, and facts. If you're writing nonfiction, the fact-checking is especially helpful. I once had an
argument discussion over a sentence in Kindergarten Rocks! that was deliberately incorrect, because that was how a five-year-old boy would say it. (I won. I mean––we came to an understanding. Heh heh.)
What does the proofreader do?
If the copyeditor looks for errors YOU made, the proofreader looks for errors the copyeditor made (well, and you). They are the last person who sees your book before it goes to print. The last thing that happens before you publish your book.
You don't want bad reviews for easily fixed errors, which is why hiring these people is important. Bad reviews for a story, or your writing…nothing you can do about an opinion. But a cold, hard grammar fact is fixable.