[1] https://en.wikipedia.org/wiki/Shahnameh
The Shahnameh is a long epic poem written by the Persian poet Ferdowsi between c. 977 and 1010 CE and is the national epic of Greater Iran. Shahnameh is one of the world's longest epic poems, and the longest epic poem created by a single author.
I one met a visiting Iranian academic just after I'd learned about the Shahnameh. I'd also read the opinion of a French scholar who thought its language was, for a modern Iranian, like Montaigne for a modern French. The Iranian woman told me that very few people in Iran actually read the book. It's very long, and hard to grasp for untrained readers. But most people know some of its stories and characters, because they are often mentioned in everyday life, and because the abridged prose books are widespread.
BTW, I don't know which editions are the most popular in Iran. Wikipedia says the Shahnameh was heavily modified and modernized up to the 14th century, when its most famous illustrated edition was created. The book most read today is probably not a scholar edition.
She makes a fair point. Reading and fully understanding Shahnameh is not straightforward. The difficulty does not primarily stem from drastic linguistic change, although the language has evolved and been somewhat simplified over time, but rather from the nature of Persian poetry itself, which is often deliberately layered and intricate *.
That said, Iranian students are introduced to selected passages and stories from Shahnameh throughout their schooling. Teachers typically devote considerable time to these texts, as the work is closely tied to cultural identity and a sense of historical pride.
* Persian, in particular, is often described as highly suited to poetic expression. Its flexible grammar and word order allow for a degree of intentional ambiguity, and this interpretive openness is frequently regarded as a mark of sophistication (difficult to master at a high-level for a layperson). A single ghazal by Hafez, for instance, can be read as a dialogue with God, a beloved man, or a beloved woman, with each interpretation leading to a different emotional and philosophical resonance. This multiplicity is the core part of the artistry.
Personally, I did not truly understand Hafez until I fell in love for the first time. My vocabulary and historical knowledge remained the same, yet my experience of the poetry changed completely. What shifted was something more inward and spiritual and only then did I begin to feel the full force of the verses.
For example, consider the following (unfortunately) translated lines:
O cupbearer, pass the cup around and offer it to me --
For love seemed easy at first, but then the difficulties began.
Books written in the 17th century or later are easy to read. Of course, the meaning of some words can change over time but that's a minor trouble. I believe Molière and Racine are still studied in school nowadays, but the first name that came to my mind was Cyrano de Bergerac (the writer, not the fictitious character).
Books from the 16th need practice, but I think anyone who tries hard will get used to the language. I enjoyed Rabelais's Pantagruel and Gargantua a lot, and I first read them by myself when I was in highschool (I knew a bit of Latin and Greek, which helped).
Before that, French was much more diverse; the famous split into "langue d'oc" and "langue d'oïl" (terms for "oui" — yes — at the time) is a simplification, because there were many dialects with blurry contours over space and time.
I've read several 11th-12th novels about the Round Table, but I was already experienced in Old French when I started, and I think most readers would struggle to make sense of it. It may depend on the dialect; I remember "Mort Artur" was easier than "Lancelot, le chevalier à la charette".
"La chanson de Roland" (11th century, Old French named anglo-normand) is one of my favorite books of all times. Reading it for the first time was a long process — I learned the declensions of Old French and a lot of vocabulary — but it was also fun, like deciphering some mystery. And the poesy is a marvel, epic, incredibly concise, surprising and deep.
Before that (9th-10th), Old French was even closer to Latin.
There's not much literature older than that, cause people preferred to write in Latin, the oldest bit in "volgare" is the Indovinello Veronese[0] which is from the 8th or 9th century and at that point it's almost latin spelling-wise, it's understandable if you're well educated but wouldn't be understandable by everyone.
In general, language changes around at the same rate all over history and geography, barring some things (migration, liturgical languages)
Resiliency is one of the weird beneficial side-effects of having a writing system based on ideas instead of sounds. Today, you've got a variety of Chinese dialects that, when spoken, are completely unintelligible to one another. But people who speak different dialects can read the same book just fine. Very odd, from a native English speaking perspective.