The essay is called "The reading process: a phenomenological approach" and although it includes quite a few big words, some of them in German, it reads pretty easily.
The introductory section of the essay quotes Sterne's joke that in Tristram Shandy he has kindly left a lot of work to the reader's imagination. Iser says that all literature does that. It's not a new observation; even Virginia Woolf saw it at work in Jane Austen. But the Virginia Woolf quotation is somehow unsatisfying. In the remaining sections, Iser goes into more detail of how the reader's imagination interacts with the text.
Part II of the essay talks of how the reader works through the text, building and modifying beliefs and expectations. He uses one of those long German terms intentionale Satzkorrelate (coined by Ingarden), which literally means 'intentional sentence correlatives', a translation it was hardly worth making. What's striking about this section is the element of chronology: the reading develops in time; it's essentially seen as a linear process. So this may be more applicable to the novel than to lyric verse for example. Iser talks about how the text has to maintain the balance between variety and implausibility. He discusses 'blockages' - twists that force the reader to change their reading, sometimes radically. Modern texts can have so many such blockages that this can be difficult.
In the short Part III Iser talks of visualisation, without really getting into detail. The point is that readers form images of what characters look like, even if not explicitly. It's an example of the input made by readers.
Part IV goes into more detail of how a text can be (in Culler's terms) recuperated. In general terms it's similar to the earlier discussion of vraisemblance: partly by reference to social codes, etc, the reader constructs a coherent and valid view of the text.
Without the formation of illusions, the unfamiliar world of the text would remain unfamiliar; through the illusions, the experience offered by the text becomes accessible to us, for it is only the illusion, on it different levels of consistency, that makes the experience 'readable'.(It's interesting there that 'readable' is more or less equivalent to Barthes' 'scriptible'.)
Part V begins with useful thoughts that reading changes the reader. It approaches a recommendation of reading precisely because of that effect. I'm less impressed by the turn it then takes. Quoting from Poulet he first notes that the ideas created in a reader's mind are not the reader's ideas, so whose are they? The author's. But apparently Poulet says that for this to happen:
the life story of the author must be shut out of the work, and the individual disposition of the reader must be shut out of the act of reading. Only then can the thoughts of the author take place subjectively in the reader, who thinks what he is not.Iser doesn't entirely buy this (and neither do I) but says there are points that "should be developed along somewhat different lines".
I feel uneasy in that I find so much to agree with in this essay. It's an introduction of course, and the detailed explanation of the theory may be more difficult to follow or agree with. It may also cover the gap I think there is here in that it doesn't deal with non-linear reading. (Actually, I realise that Stanley Fish's reading of Milton is largely based on a linear reading. It has limitations.)
I suppose the other main concern with the approach as laid out in this essay is that it's so anti-theoretical. The danger is that an absence of theory may hide an unquestioned ideology. I'm sure that attack has been made by others and I'll encounter it at some time.