A copy editor or line editor focuses on improving sentences and grammar, as well as suggesting other ways of rephrasing ideas, and providing comment on anything else they pick up. Non-fiction books often use multiple styles, for example, chapter headings, sub-headings, action points, bullet points, call-outs and examples, so these will be examined at this phase.
I cannot speak for all editors, of course, but I know I and many of us would be open to editing just part of your book at first, say, a particular chapter or two that you sense needs a lot of work. That is often enough to show you mechanical errors that you tend to make repeatedly. You can then go through the entire manuscript yourself looking for as many of the same kind of errors as possible. You will sharpen your basic writing skills in the process, and it will pay off in all future projects.
The single most decisive factor should be the quality of the edit itself: Will this editor help improve your book? In order to assess this, nothing beats a sample edit (even if you have to pay for it). Testimonials and credentials (such as experience at a major NY publisher) can help you narrow your list, but you should submit the same sample to a variety of editors and compare their work side by side. Some editors will push too hard, and some won’t push hard enough; some will simply “get” your writing, while others will seem to speak another language entirely. In my experience, the editor’s résumé is far less revealing than the quality of the sample edit. In fact, the higher an editor’s pedigree, the more reluctant the editor might be to provide a sample edit. Don’t be afraid to insist: no one should expect you to invest in a car without a test drive!
In fact, Pressfield writes, “Resistance is experienced as fear; the degree of fear equates to the strength of Resistance. Therefore the more fear we feel about a specific enterprise, the more certain we can be that that enterprise is important to us and to the growth of our soul. That’s why we feel so much Resistance. If it meant nothing to us, there’d be no Resistance.”