Michael Crichton on the importance of editors (in The Paris Review, 1994):

In my experience of writing, you generally start out with some overall idea that you can see fairly clearly, as if you were standing on a dock and looking at a ship on the ocean. At first you can see the entire ship, but then as you begin work you’re in the boiler room and you can’t see the ship anymore. All you can see are the pipes and the grease and the fittings of the boiler room and, you have to assume, the ship’s exterior. What you really want in an editor is someone who’s still on the dock, who can say, Hi, I’m looking at your ship, and it’s missing a bow, the front mast is crooked, and it looks to me as if your propellers are going to have to be fixed.

He was talking about writing, but this also captures the essence of a good designer/art director relationship. Working in isolation, it’s all too easy to lose sight of things. So many run-aground covers have been saved by an art director coming back to me with simple – and in retrospect, obvious – feedback. Sometimes it’s just a tweak of shade or size, but that extra perspective is invaluable.