Check out freelance websites like UpWorkor even a great site called Scribendi. (Warning: if you source an editor from these sites, make sure you hire another, professional editor to go over it afterward. There is no way to know what you are getting otherwise. Just because the draft comes back better than it was before, does not mean it was well-edited!)
If you’re a first-time author and have never hired an editor before, I would not recommend hiring an editor without first getting a sample edit. An editor can have a great resume and speak eloquently on the phone, but the real test of their skills is how they edit your book, and the sample edit is a quick, free way to find out. It’s standard for a freelance editor to offer a free sample edit unless they are exceptionally well-known.
There are different levels of editing—developmental, copyediting, and proofreading. I’ve found that authors are often willing to pay good money for a developmental editor, someone who walks by their side and helps to shape the book, but when it comes to copyediting and proofreading, especially if and when an author has had a developmental editor, suspicion arises as to the value or merit of these more drilled-down types of edits. Don’t let this be a blind spot; they’re important—on par with a developmental edit for sure.
The single most decisive factor should be the quality of the edit itself: Will this editor help improve your book? In order to assess this, nothing beats a sample edit (even if you have to pay for it). Testimonials and credentials (such as experience at a major NY publisher) can help you narrow your list, but you should submit the same sample to a variety of editors and compare their work side by side. Some editors will push too hard, and some won’t push hard enough; some will simply “get” your writing, while others will seem to speak another language entirely. In my experience, the editor’s résumé is far less revealing than the quality of the sample edit. In fact, the higher an editor’s pedigree, the more reluctant the editor might be to provide a sample edit. Don’t be afraid to insist: no one should expect you to invest in a car without a test drive!

Line editing is an intensive structural edit that focuses on the finer aspects of language—the flow of ideas, transition elements, tone, and style. Your line editor will take a critical look at your manuscript’s writing flow, language usage, character development, and more, and make suggestions that ensure that you’re communicating your story effectively while maintaining your voice.
Stephen, you’re right, and I think much of the confusion about editing and its pricing stems from the fact that editors offer a range of services (in addition to a range of experience). It might be best to view editing on a spectrum from broad (developmental) to detailed (copyediting and proofreading). And it’s always best to have as clear an idea as you can about the kind of editing you believe you need.
What an editor does is discover your characters, your situations, and your images without seeing any of the creative process that brought them to life. Where you might see all the crossings-out and labors, all the accidents and decisions, the editor sees only a page. This is the clarity you need, and you can never achieve it for your own writing, simply because you envisioned it first. The editor will tell you what an attentive, an educated, and, most importantly, a new reader will experience while reading your book.

Book editing is both subjective and not. There are rules, and yet the rules of writing are often intentionally broken. You want to find someone who knows the rules, who will fix the writing, and who’s flexible enough to know that voice is sacrosanct, and that editing out colloquialisms, slang, and humor is a no-no (unless it’s overdone or not working, that is).
Editors are an integral part of content generation at any level. A quality editor is responsible for the final, polished look and feel of any written content produced by a client. Freelance editors provide a flexible and talent-driven way to manage multiple products in a professional way. They can be used for a wide variety of projects, from editing for syntax and grammar on regular blog posts to providing structure, pacing, and style feedback for nearly finished novels.
Regarding your last question, I know of no editor who gives a money-back guarantee for occasional errors that happen to slip through or for a different opinion by a later editor. I’m afraid such things are just the nature of the publishing beast. But by having a shorter portion edited first, you will get a decent feel for whether someone is the right editor for you, and will be able to be confident in his or her work on your entire book.
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