I'm interested, reading Italian, in the words I don't need to look up. Calvino has, I've discovered, a difficult vocabulary, something I might not suspect, reading him in English. Lots of the words he uses are not in my Italian desk dictionary and I need to look them up online. But looking everything up would take all the fun out of reading him, so some things that I can see from the context or their part of speech are "just" illustrative detail I don't look up, limiting myself to the words that would prevent me following his argument, in this essay I'm reading on the painter Giulio Paolini.
I was thinking about this reading Shakespeare's Sonnet 73 ("That time of yeeare thou maist in me behold / When yellow leaves, or none, or few doe hange / Upon those boughes which shake against the could, / Bare ruined qiers, where late the sweet birds sang, etc.") This is all detail: the argument is in the detail (and the clincher couplet, never to my mind the best part of a Shakespearean sonnet, too perfunctory).
I guess what I'm saying (to myself) is that there are different kinds of writers, and considerable variations in the linguistic structure or fabric of their texts. Calvino, in some of his work, might be more like Donne (in say the "Nocturnall"), all lucidity and thinking. So I can skip a few words in a paragraph of illustrative detail and still follow his argument, which is my main interest in this essay (reading Invisible Cities might be another matter). But if I weren't interested in the description in Shakespeare's Sonnet 73 I might as well not bother reading it. It is so descriptive, in fact, that I can imagine a visual equivalent to every line--as in a child's picture book. Try doing that with the Donne.