In December I posted about working with illustrators, referring you to Marlo Garnsworthy, an editor and illustrator. More recently, she has done a two-part piece on working with freelance editors.
In Part One she discusses pre-editing issues, among them, what to look for in an editor and the difference between developmental and copy editing. In Part Two she writes about what happens while an editor is at work.
I can't say enough about the importance of developmental editing. Many people outside writing don't know what it is. (Read Marlo.) Many very new writers don't know what it is or understand why they need it.
Quality illustration and editing are the hallmarks of professional publishing. When self-publishing began, it took a big hit as a whole because of lack of attention to these two things. The self-publishing community recognizes that. Editing is something all writers, self-published and traditional, need to understand.