For my book, Career Change, I gave an early copy to people working in my department in the day job who I knew were dissatisfied with what they were doing. They came back with questions and suggestions for what to include as additional material. For The Healthy Writer, we asked medical doctors to read it as a sense check (even though my co-writer, Euan Lawson, is a doctor!)
Editing is one of those skillsets that many people claim to do well but which few actually do. And while it’s probably the most important service an author can solicit (second only to book cover design), it’s often undervalued. Furthermore, most authors have no idea how to assess an editor’s work, and the result can be catastrophic, ranging from an editor who introduces new errors to an editor who changes the intention of your writing.
I’d add that for UK writers, the Society for Editors and Proofreaders (SfEP) Directory is the best place to search for editorial professionals. Each editor has been vetted for professional competence through the Society (you have to be assessed to become a Professional Member or Advanced Professional Member), so it’s a pretty trustworthy source: http://www.sfep.org.uk/directory/directory.asp
Note: On the spreadsheet, the editor’s total cost will be automatically calculated once you insert your total word count and the editor’s per-word rate. If you’re given a per-page rate, you can calculate a per-word rate by assuming the industry standard of 250 words per page, e.g., $3 per page equals $3 per 250 words. Dividing 3 by 250 equals $.012.
Start with a google search and see what comes up. Click on a handful of options and see if you can find an editor who is specialized or who has edited books similar to your own. An easy way to do this is to open just a few of your favorites in tabs and check out each editors’ website and about me section. Take a look at their testimonials. See if any stand out to you.
Another online editing service is Scribendi, which has over 300 experienced and educated editors. They've been around a long time. With this service, you decide how fast you want the book edited and how extensive the edit should be, then they match you with an editor. You don't ever actually meet the editor, but rather work through their customer service team. Their prices depend on your requested turnaround time and word count. You can get a free quote here.

In addition to the individual editors listed above, the Reedsy marketplace is full of vetted professional editors for every editing level and genre. They screen their editors and only allow the best of the best to be listed on their website. I'm a BIG fan of them because they have made the process easy for me to find the best book editors, book cover designers, and even writers to help me with my projects.  Talk about awesome!
The U.S. Bureau of Labor Statistics states that employment of editors is expected to have little change up to 2026, as print media continues to face strong pressure from online publications. Editors who have adapted to online media and are comfortable writing for and working with a variety of electronic and digital tools will have an advantage in finding work. 
That said, I don’t recommend going to publication, particularly if your book is intended as a revenue-generating venture or as a vehicle of professional promotion, without making sure that someone with an eye for detail has gone through the entire manuscript and ruthlessly noted every typo, every spelling error, every misplaced comma, and every ambiguous sentence.
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.
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