Yeah, I know I’ve written about editing, particularly developmental editing, before. But it is one of the most important aspects of professional-quality writing, so I’m going to mention it again. At Prolixme a new, unpublished writer discusses developmental editing. She is interested in whether or not it is necessary before submitting a manuscript to agents. I’ve heard this question before over the last few years. But only in the last few years.
Having been through the the developmental editing process with eight books, myself, I think it's pretty clear when a book I'm reading hasn't had developmental editing or has had the wrong kind. I've read a self-published book in which the title only connected with the last quarter of the book. If the last quarter of the book was what the story was about, it took way too long to get there. A developmental editor would have pointed that out. I've read a self-published book that described itself as YA in which the YA character disappeared for a third of the story. That portion focused on an adult character we hadn't seen before. The author's note sounds as if there may have been developmental editing for that book. I wonder if the editor was familiar with YA. Long, unnecessary passages...characters who sound too much alike...awkward point-of-view switches...all these things should be pointed out by a good developmental editor.
I don't think developmental editing is necessary before submitting to agents, but it's certainly necessary before publishing. From what I've read, knowledgeable self-publishing writers realize this.