"If editing has become a luxury in today's world, because of the accelerated pace of publishing and the pressures on editors to spend their time acquiring, it is no less important in the life of a book. No matter what anyone may ascertain about an editor's abilities, it is finally the writer who knows whether the editor has a gift for language, an understanding of structure, a grasp of the dynamics of plot, pacing, tension, and resolution. Only the writer knows if his editor edits. The best editor is a sensitive reader who is thinking with a pencil in her hand, questioning word choice, syntax, and tense. An editor is someone who probes the writer with insightful questions, who smooths transitions or suggests them where none exist. A good editor knows when the three pages at the beginning of a chapter are throat-clearing. Start here, she'll mark in the margin. This is where your book begins. And she'll know when you should stop, spare your reader being hit over the head as if your point were a two-by-four."
From The Forest for the Trees: An Editor's Advice to Writers by Betsy Lerner.
If you're interested - as I am - in the subject of editing, do read the article The Art of Editing, the Paris Review interview with editor Robert Gottlieb, from which the following extracts are taken:
"Editing is simply the application of the common sense of any good reader. That’s why, to be an editor, you have to be a reader. It’s the number one qualification. Because you could have all the editorial tools, but if you’re not a responsive reader you won’t sense where the problems lie. I am a reader. My life is reading. In fact, I was about forty years old when I had an amazing revelation - this is going to sound dumb—it suddenly came to me that not every person in the world assumed, without thinking about it, that reading was the most important thing in life. I hadn’t known that. I hadn’t even known that I had thought it, it was so basic to me."
"What is it that impels this act of editing? I know that in my case it’s not merely about words. Whatever I look at, whatever I encounter, I want it to be good - whether it’s what you’re wearing, or how the restaurant has laid the table, or what’s going on on stage, or what the president said last night, or how two people are talking to each other at a bus stop. I don’t want to interfere with it or control it, exactly - I want it to work, I want it to be happy, I want it to come out right."
See also: Points to Ponder