Authors I work with often say to me: "This manuscript is pretty clean, I've rewritten it a dozen times." Most often, after that, I come up with dozens of errors--sometimes the number is in the hundreds. An author might lay the blame elsewhere…"I've had it read by quite a few friends." Same result.
While traveling the last couple of weeks, I've been working on a manuscript that was co-written by a fine writer and editor; I've known her for years and her attention to detail is only equaled by her excellent research. This new manuscript was quite compelling, with a clear story arc, but I found many an error--both typographical and grammatical--in it.
For those authors who think software tools like Spellcheck are the answer, remember that such techno-tools can't make the correct choice between "he peeked through the curtain" and "he peaked through the curtain"… And though the second choice might be accurate in some erotic or pornographic writing, it definitely won't work in most books or stories.
And the problems are not limited to these kinds of errors--things that could easily be caught by copy-editors or bright-eyed English majors. Often it is the structure of the book that needs help, too. Opening chapters are usually the most flawed, but I've also found chapters deep into manuscripts that needed to be moved, heavily cut, or even completely eliminated.
The problem is, that all writers--even editors--need editors…We are all too close to our own writing to see its errors and flaws. I would definitely hire an editor to read anything I was publishing.
The bottom line: If you want your writing to shine, hire an editor. Trust me.