But if you do...
I decided to hire an independent editor, so I thought I'd share that experience.
NEPOTISM ALERT: A highly respected author, a family friend, has agreed to look at my ms and possibly pass it on to his agent, pretty much the A-list of the A-list; I felt a bit as though I were scheduled for my first major-league at-bat in the World Series. I know she won't look at it, but I'd at least like to impress her junior assistant. I spent two years paralyzed because the I was intimidated by the mere name of this woman.
After consulting my contact, I decided to have someone take a look at it; I wanted (and still do) to make it bulletproof before I even sent it to him.
I did *not* go with anyone I found on the Internet. I thought about the editor I most enjoyed working with in the past (on a short piece); I felt she really understood what I was trying to say and communicated her ideas to me very clearly, in language that fit my brain-scheme. I e-mailed her, and though she wasn't able to take on any projects due to family commitments, she e-mailed a few of her contacts, and I wound up sending my ms to a former New Yorker copyeditor (I think she worked on features) who has significant book-editing credentials to her credit as well.
She agreed to a simple read, which included a lengthy phone consultation during which she made suggestions and I asked questions. Though I was deflated by her response - well-written but not marketable without a 180-degree change in tone - I felt I'd received high-quality advice for a reasonable amount ($320 US).
Some other thoughts:
-I am usually very good with grammar, spelling, sentence structure, etc. I went over my ms with a fine-toothed pencil, and I had a couple of other people read it, too. If this isn't your strong suit, there's no point in having it copy-edited until it's structurally sound.
-By the time I sent it to Ed #1, the ms was absolutely the best I could've possibly made it. There wasn't a word in there that I hadn't scrutinized; I put the whole book as well as each chapter through my story structure test, made sure continuity was correct - in short, it was what I might've sent to an agent ten years ago, back when editors at publishing houses had time to edit.
-Now. I plan to implement as many of those revisions as make sense (a few don't, at least not right now); I'll again take it as far as I can, and then I plan to work with another editor (slightly less expensive, but in more line-editing detail) I'm not turning it to my contact in until I can bounce a quarter off it.
-I have several regular readers. Because they're friends and family, they a) give me more leeway than a stranger might, and b) fill in the blanks if crucial information is missing. Still, they all have good eyes and can catch continuity errors, inconsistencies in voice and characterization, etc. So I don't see any point in hiring an editor until you've gotten as much as you can - for free, or the price of a home-cooked meal - from the people you know
!!
-Go with someone you trust, someone who (as has been said above) has a major track record. And don't waste your money by hiring someone too soon. The 'read' was a great way for me to learn what was on track and what wasn't. For feedback from a woman of her stature, $320 was a steal.
-Finally, I had to wait just over a month for this woman to get around to my ms. That's actually a very short time, but know that the best editors are swamped and giving detailed attention to their current projects, the same kind of attention you'd like them to pay to yours. With this second editor (who is also a friend), I've been able to estimate when my revisions will be done and ask if she can save me some space around that time.
There are a whole lot of scammers out there, but there are a few genuine editors who can help your final product become tight - but first find out if it's worth the extra time and money. I have confidence in my project because one of the main characters is relatively famous; I now see that I have to embrace that fact rather than try to hide it.
Hope this makes sense. It's very late, and I'm falling asleep at the keyboard.
As always, YMMV.