A developmental editor helps you shape your book. Rather than focusing on a line-by-line edit (though some developmental editors also do this), the goal is to focus on the structural organization of the book – does the narrative flow coherently? Are there plot holes? Are terms explained appropriately for the audience?
A developmental editor will help you trim areas of your book that are too long, and punch up details that need highlighting. They are sometimes called “book doctors”.
But do you really need a developmental editor?
Most books need another set of eyes to objectively review the content. It’s a rare author who can write a perfect book without feedback. A developmental editor doesn’t re-write the book, but helps its evolution, in the writer’s own voice.
Alan Rinzler, a consulting editor with many years in the business, lays out what an author should expect from her developmental editor here.
You can find listings for developmental editors at The Editorial Freelancers Association and Writers and Editors.