Developmental editing is the only kind of editing that can change what your book is. Everything else changes how it reads.
Your editor works through the whole manuscript looking at architecture. In fiction: whether the story earns its ending, where the middle sags, whether characters want things clearly enough to drive scenes, whether the viewpoint is disciplined, what the reader knows at each point and whether that is deliberate. In nonfiction: whether the chapters are in the order a reader needs rather than the order you learned them, whether each chapter has one job, whether the evidence supports the claims, and where you are explaining something the reader already accepted three pages ago.
You receive two things. First, an editorial report typically fifteen to twenty-five pages that lays out the manuscript's structure, identifies the specific problems in priority order, and proposes concrete solutions rather than describing symptoms. Second, the manuscript itself with margin queries at the exact places where something is going wrong, so the report's abstractions have addresses.
Then you revise. This normally takes authors between one and four months, and that time is not wasted it is where the book actually improves. Your editor is available for questions throughout, and a follow-up call is included once you have read the report and had time to sit with it.
This is the most demanding service we offer, for you as much as for the editor. It is also the one that most reliably changes how a book is received.
