As an editor, I can catch manuscript errors before they get self-published. As a publicist, I receive too many self-published and small press books riddled with errors from cover content to basic spelling mistakes.
Many writers fall into the "Well, even if it's not perfect, my story is still there" frame of mind. They don't understand that spelling, grammar and format errors not only turn off readers, they turn off media professionals who refuse to interview an author whose book has editing errors. If there's a mistake on the cover, it can end up in their trash.
Beyond the basics of manuscript editing, there is the far trickier territory of allowing an editor you trust to help you shape your story structure and characters to their best possible advantage. Newbie authors fear the editor will change too much of what they love in their writing, and self-published authors without solid know-how believe they either "know it all" or rely on spouses to edit. The latter works fine if your spouse has the credentials for editing, although an English degree may not be enough. Academic writing goes by somewhat different style standards.
So as not to re-invent the wheel, I am re-printing these beautifully articulated tips from Jerry Jenkins' website at https://jerryjenkins.com/self-editing/:
Why Publishers Reject Your Manuscript After Reading Just Two Pages
- Editors can tell within a page or two how much editing would be required to make a manuscript publishable; if it would take a lot of work in every sentence, the labor cost alone would disqualify it.