Think of yourself as a race car driver and your book as the high performance vehicle. You’re in control of its destiny, guiding it through every hairpin turn. Regardless of how accomplished the driver, how excellent the car, a racer would never enter the race until a mechanic certified it ready to drive. An editor is your mechanic, the professional who gets under the hood and gives your writing the green flag. Regardless of how many times your spouse has read the manuscript and your English teacher aunt has proofread it, do not submit it to an agent, editor or contest until an editor has reviewed it.
Publishers require authors to submit faultless manuscripts. As more authors publish their own work, they need to be aware that POD (print on demand) publishers print what you submit. To not edit your work is to be penny wise and pound foolish after you have spent ten years or more writing your book.
A good editor is ideally impartial–someone with skills, and an established client list. Fees may be structured hourly or per page, and a deposit may be required. Some books can be edited on a chapter by chapter basis, but always have the completed book reviewed by an editor.
Remember, you are in the driver’s seat and have the final say on what gets added, deleted or changed. An editor should share your vision and be committed to helping you achieve it, not mold you to their own writing style or perspective.
Let agents, editors and publishers know in your query letter you have had your manuscript professionally edited. It shows them you are committed to providing quality work.