Think how many times you've received a reply to an email you sent out, only to noticed that your original text contained errors. It turns out that editing one’s own writing is remarkably difficult. Once they are initially missed, errors tend to become "invisible" to a writer when they review their work. Our eyes skim past them. A lot of new writers make the critical error of self-editing and think they are done.
If the publishers of writers like James Patterson. Issac Asimov, Sue Grafton, Robert B Parker, Raymond E. Fiest all require an editor to review and check their work. What makes any of us first time writes believe we are better and don't need an editor.
My co-author and I had our self-published Tyranny series picked up by a publisher in Las Vegas. What was the first thing the owner did? He tore out the first chapter (literally). We had to create a whole new chapter starting in a different place. Next he took a scene from book one and had us move into to book two. Next he underlined all the placed where we did tell and not show. We sat together in a marathon session over a weekend and removed all the places he raised objection. Now the company editor is taking her turn and bringing up very good points about character development, scene description, and repetition in the story. The process is frustration at times but necessary.
I'm doing a last read-through of The Adam Eradication. I printed the entire manuscript, got away from my computer and started reading. While the manuscript is complete, and was edited twice, this read is pointing out a boat load of typos made while removing my telly pros and replacing them with pros that help the reader to see the story.
This brings to mind an elderly woman I met. She wrote a book and was very anxious to get it published. So anxious in fact she didn't want to bother having an editor read it at all. This was huge mistake, made by those who decided to self publish.
Concerned about her book I request she allow me to send the first five chapters to the woman who did the first edit on my book. She went through the pages as a courtesy. What this editor returned shocked to woman into taking a second look and she paid the editor to run through her manuscript.
She received her red-lined manuscript back entered the edits and corrections. This writer figured she was done. I tried to convince her to have someone outside her family read through it one more time. Her reply, I made all the changes, it's ready to go.
She had 500 copies printed. After receiving cases and cases of books, her son took one home to read. He returned it to her three day later, red lined, pointing out errors on almost every page. Her concept is a good one, but instead of book worth reading she had boxes of books that are good for little more than starting a fire.
My first editor did the work and I made the corrections. But The Adam Eradication, being my first book, I found what I considered holes in the story and proceeded to plug them. Enter a good friend (enough of a friend to tell me the truth) she did the second edit. The first thing she did was cut the last 10 chapters (48K+ words). Her analysis. Your book ends at this point. A good piece of advise. I used those 10 chapters to start the second book. (Never trash anything you've written, archive it. A good idea can be reworked to fit somewhere else)
Throughout the edit she deleted whole paragraphs as redundant. Some writer suffer from this problem some do not. I was anxious to get the story to press and rushed though the last edit. After publishing the book I gave several copies away. My friend's wife read the book and sometime later handed me her copy. She said "I love you story but --" The book was marked with over forty sticky tabs each on a typo. Though minor typos, I learned my lesson. Fifty fire starters.
Editors cost money. Not as much as publishing an unreadable book. This mistake marks you as an armature, not serious about your craft. This kind of attitude will stop readers from ever considering reading any future work you publish, no matter how much you improve. And without someone finding your story compelling enough to look past your mistakes, no published will take you seriously in the future.
What can an editor do that I can't? What can an editor do that you can't? An editor will read what you wrote, not what you think you wrote. A good editor will find your inconsistencies, help you with the language appropriate for your genre. For instance a steam punk novel's use of nouns, pronouns, adjectives, etc will be different from those used in a science fiction novel as will a fantasy fiction novel will differ from SF or romance. Next, an editor will point out your flaws in the construction of paragraphs, scene breaks and poor character development. Some writer's characters all talk the same. A good editor will point this out and help you see how to vary your character's personalities. A good editor will tell you when you've shifted you POV(Point Of View) If character A is you POV character you cannot have character's B thought in a scene or describe a scene through character B eyes. You can if your novel is written from an omniscient POV, but few good writers use this POV anymore.
An editor will spot problems with spelling, punctuation, and grammar that can escape your notice.
Does the phrase familiarity breeds contempt. Well, familiarity with your subject can result in explanations and descriptions that are incomplete or confusing to readers. What you see in your minds eye is always what you've put on paper. An editor will ensure that your copy is intelligibly and concisely written.
You may also ask yourself why an editor is necessary when your word processing program already has spell checking capabilities. Spell check, it turns out, is only partially effective at catching problems. For example, homonyms (e.g., council/counsel) you won't find these flagged. Words used nonsensically are regularly accepted by a spell checker. These programs have nothing to say about confusing, ones needlessly wordy, jargony or telly prose.
One of the biggest helps is to join a writer's group. You read your work aloud and they critique your writing. Be advised, while most groups offer good non threatening environment, you need a thick skin, because it is still hard to hear your writing evaluated by others.
You took the time and effort to create a good story. Give it the polish and attention it deserves.