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!
The key historical factor that enabled the U.S. to become a world economic and military power after WWII was government leadership in funding high-risk research and development (R&D) and the education of generations of scientists and engineers. Since 1967 the Federal government has steadily reduced funding in these areas and now is a relatively small player in our national R&D enterprise. This puts our future prosperity at risk. The book goes into great depth as to the factors that have caused this change in government policy.
Regarding your last question, I know of no editor who gives a money-back guarantee for occasional errors that happen to slip through or for a different opinion by a later editor. I’m afraid such things are just the nature of the publishing beast. But by having a shorter portion edited first, you will get a decent feel for whether someone is the right editor for you, and will be able to be confident in his or her work on your entire book.
Another great resource is Reedsy; a platform for finding freelance editors, writers, and all things book related. You can also use the Editorial Freelancers Association to help narrow your search. These websites are fantastic because they vet their members, meaning you will only be viewing the best of the best when it comes to editing. Keep in mind, Reedsy prices will typically be higher than going to an editor directly because the website, rightly so, takes a commission (from both the editor and the author) for finding, vetting, and then managing your contract.
Working with a professional editor can be one of the most rewarding experiences in an author’s life. You’ll probably learn more about the craft of writing — plot, characterization, dialogue, worldbuilding… — than if you took an creative writing course. However, as is always the case in creative endeavours, critique can be hard to accept. This is why you should seek an editor who’s not only experienced in their field, but whom you’d feel comfortable receiving feedback from. In the words of one of our Reedsy editors:

Experience: Experience, including internships at publishing houses and work in other media—such as newspaper or magazine editing—also is important for prospective book editors. Moreover, connections in the publishing world, whether to another editor or successful writer, also can help your chances of landing a job as a book editor. If you are interested in editing books that deal with a specific subject, such as fashion or food, you should have formal training or work experience in that area to increase your chances of getting hired. To grow your network, you can join professional organizations such as the Editorial Freelancers Association (EFA).

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.
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