At Sulis International, we alwasy stress to our authors the importance of an outside editor. No writer can catch all the problems with their own work. We have an in-house proofreader for typos, basic grammar, and punctuation, but we require our authors to send us clean manuscripts (that’s one way we save money and offer higher royalties to our authors.)
This article below stresses the need for multiple editorial rounds. Here’s an excerpt:
Here’s the problem. The vigor required of literary practitioners prior to this flood of self-publishing has now been severely diluted. Most self-published authors send their books to print prematurely. It’s a nightmare for reviewers, journalists and competition judges, who are deluged with titles by authors who are far better at promoting themselves than they are at the actual writing of their books. Talk about putting the cart before the horse! I would estimate that 95% of self-published books go to print with not nearly enough research and editing.
Never let your book go to print without it having been properly vetted by professionals. This does NOT mean Aunt Betty, the retired English teacher. You can let her comment on your early drafts, of course, but what you consider your final draft should go to a professional editor, who can give you honest feedback on what you have actually accomplished, rather than what you think you may have accomplished. This is what happens when established authors submit manuscripts to their editors at the big publishing houses. Sometimes they get torn to bits and have to take the book “back to the drawing board.” You MUST put yourself through the same painful testing if you are serious about your work.
Authorship is not easy and it is not cheap. Authorship is a profession and you should never think about it any other way. Editorial support is the key to an author’s success.