One important question to establish the answer to early on is what level of editing your work needs. Do you need a developmental editor, who can help you work on the structure of your book? Do you need a copy editor, who will make sure it follows a style guide and reads consistently? Or are you at the final stage, where it’s just a proofread to check for remaining errors?
But another important question is what the editor specialises in. There are many different types of writing – fiction, which subdivides by genre; self-help books; technical books; academic writing, and many more. And each editor will have their strengths and weaknesses and preferred types of project.
Personally, I’m well-versed in issues such as plot development, story arcs, and point of view issues, but show me a list of citations and I’ll struggle. For other editors, who are used to working on highly academic or technical texts, fiction might be their weakness.
Each editor will have their usual language to work in, as well. Believe it or not, there are definite differences between UK English, American English and Australian English, for example, and while some editors might well work competently in more than one variety, others will prefer to work in the one most familiar to them, while some localisms might well be overlooked or misunderstood if your editor is not familiar with them.
So when you’re looking for an editor, remember to check what type of writing they are used to editing, or prefer to edit, and then you’re more likely to find your perfect match.