Developmental or substantive editing is the process of revising the structure of a book or other document. The word “development” applies because some people hire a developmental editor to help create a better book from the beginning, rather than to make improvements afterwards. In most cases, however, the author has completed at least one draft of the manuscript before he (or his publisher) goes in search of help.
The main issue with substantive editing is to rearrange the content in order to create and maintain what famous ghostwriter Claudia Suzanne calls “slinky flow,” the momentum that carries the reader automatically from beginning to end of the book like a Slinky going downstairs. Sometimes this requires major readjustments, severe pruning, or additions and clarifications, making it closer in spirit to ghostwriting (or at least rewriting) than editing.
Yet substantive editing is not always substantial in the number of changes made, only in their effect.
Some authors (and many publishing houses) prefer to have developmental editors make detailed “queries,” that is, lists of suggested changes and questions for the editor, either as a separate document or using Microsoft Word’s comments feature. At that point, the author decides which changes to make and how. Other authors prefer to have the editor make the changes (again, with some kind of “versioning” in place so they can return to their original if they have to) directly on the document.
I’m happy to work either way, but it’s usually faster for me to just make the changes than write a commentary about why I think you should make them.
The natural next step for an author after getting a substantive edit done is to hire a copyeditor. While I will certainly correct things like grammar and spelling in the course of doing a substantive edit of your book, one of the main purposes of copyediting is to make sure your manuscript complies with the house style used by your publisher or company. That covers things like headings and subheadings, numerals, hyphenation, and citations.
There are people who are much, much better at that than I am. They know the Chicago Manual of Style backwards and forwards. My personal recommendation if you’re looking for a copyeditor is Hilary Powers, but if she’s busy (and she probably is), check out the Bay Area Editors’ Forum or the Independent Book Publishers Association for a referral.