Here's Paul Fry on Wolfgang Iser, from the fourth lecture of his Yale course in literary theory (online in its entirety here; thanks to Mike Johnduff for the link):
"'One text is potentially capable of several different realizations, and no reading can ever exhaust the full potential, for each individual reader will fill in the gaps in his own way.' And this, of course, brings us to the issue of gaps and the role that they play in the act of reading as Iser understands it.
"A gap. It's an interesting term. I don't actually know whether Iser — to be Hirschian — means what I'm about to say about gaps, but plainly a gap is an abyss, it's a distance between two points. But what's really interesting is that we think of spark plugs, we think of 'gapping' a spark plug. I don't know if you know how a spark plug works, but for the electrical current to fly into operation in a spark plug the two points of contact have to be gapped. They have to be forced apart to a certain degree. Too much: there's no spark. Too little: you short out; there's no spark … And it seems to me that that 'a-ha!' effect of reading, the movement back and forth across the gap between the reader and the text, can be understood in terms of a spark — right? — as though the relationship between the reader and the text were the relationship between the two points of a spark plug." (Track 04, 26:18 - 27:57)