Developmental book editing is a type of editing that focuses on the overall structure and development of a book, as opposed to line editing, which focuses on the individual sentences and paragraphs. Developmental editors work with authors to help them organize and develop their ideas, and they also offer feedback on the overall direction of the book.
There are different levels of developmental editing, from general feedback on the overall plot and structure of the book, to more in-depth feedback on individual scenes and chapters. Developmental editing can be a very involved process, and it can be helpful to have an editor who is familiar with both the book's genre and the author's style.