Every manuscript needs to be edited, and not just by the author. Sure, self-editing is an important step in manuscript preparation, but it’s not the final step if you want to be proud of your published work.
We authors simply can’t catch all of our own mistakes. We tend to read what we think is there, rather than what is actually there, which makes us terrible proofreaders. Plus, truth to be told, we may not have a perfect grasp of grammar and usage.
That’s where a second set of eyes can be invaluable. A friend, associate or, ideally, professional editor can catch mistakes we’d never see, and elevate the level of the final result.
If you’re being traditionally published, your contract comes with an editor. But if you’re self-publishing, editing is just as important, because now Amazon allows readers to report errors in self-published books, and takes that into account in the rankings that govern your sales.
There are many levels of editing, and the cost can be anything from free to a thousand dollars or more. As with anything, you tend to get what you pay for. This excellent article describes the different types of editing, and provides many resources for finding an editor who matches your needs and budget: