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.
Developmental Editing helps you fine tune the sequence and the order of your book. Have you presented the information in a format that makes sense to the reader; does each piece of information build upon previous material? If you’ve had a few people read drafts of your book and they’re confused, you need developmental editing. A developmental editor may even be able to help you uncover features in your work that will create a powerful experience for your readers.

You know, Rob, I’ve learned people offer their services for pay for a variety of reasons, and they live in a variety of circumstances. Some do it just to make a little side money, because they love the creativity…it’s not everyone who’s doing it because it’s their main livelihood. Others are new to the field and don’t feel they can command more. And sometimes, maybe you connect with someone who’s affordable, but they still do a great job.
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Second, consider getting a critique before you decide whether you need to pay for a full copyedit. I (and probably many other editors) will read your manuscript and write a several-page critique for much less than the cost of a line-by-line edit, and if your mechanical skills are reasonably good, this may be sufficient to help you get it into adequate shape for your publication plans.
Obviously there is a big difference. I actually started with screenplays (with some mild success) partly due to hearing that as long as you have a good story, the grammatical errors won’t be a big deal. Well, that was not true. Maybe if a producer liked the story or I was already established but every contest I entered always came back with feedback about the grammatical errors. Even if I won or placed as a finalist, I always received critique on that. Very little is spelling error, it was more about structure, etc.