Developmental editing is what most professional writers think of when they use the word “editing.”
It isn’t fixing spelling and grammar. Rather, it’s offering guidance and feedback to help you develop your book. A developmental editor’s job is to look at your book from the reader’s point of view, and ask some key questions.
If it’s a novel, or work of fiction, these may be, “does the plot make sense here?” “Are the characters vivid?” “Are you using dialogue correctly?” Is the pacing right? Are you telling the reader what to think, rather than letting the reader discover his or her own ideas about your book?”
For a nonfiction book, the questions might be, “Is it structured well?” “Do the arguments make sense?” Are the examples sufficient? Are there holes in the research? Are you telling the whole story? Does the book need to be cut, or added to? Are you being wordy? Are you being vague?
In short, the developmental editor asks the hard questions that you may not have the perspective to ask yourself when you’re writing your book. Typically, the Book Doc will write you a letter asking questions and making suggestions aimed at helping you get the big things right.