I share many of the author's delights. I would add another source of "short little difficult books": old books, written in an old form of your modern language.
My native tongue is French, so in my case it was Rabelais' "Gargantua" (~1500) and "La Chanson de Roland" (~1100). Both books require some motivation to get used to their respective languages. After a while, I could read almost fluently Rabelais' prose ; it was immensely funny, coarse and impudent. Roland was harder to grasp, I had to use a specialized dictionary, but it's a concise and epic tale, and I read some verses so many times I ended learning them by rote.
Even translated, some short books from a very different culture far in the past can be challenging. For instance "Eugene Onegin" or "Gilgamesh". As a counterpoint, "The art of war" is an easy read, though written 2300 years ago in a small kingdom of China.
A last comment: the author conflate books that are difficult because of a technicality in their writing, and those that are strange in their story. The translations of Abe Kobo or Kafka I've read had nothing difficult in their words, but the surrealist plots were very unsettling. In "Pedro Páramo" the reader feels lost in a harsh world and unsure of reality. Meanwhile, Perec's "La disparition" or Becket's "Molloy" are more about style tricks.
I'm sure Gargantua and Pantagruel is tough in French, but I'd like to recommend it in English translation to people who don't speak French where it's just a lot of good fun. Plenty of older French works (or a lot less old, like Zola) that have been translated to modern English are just good, easy reads.
I'd assume that in French they're tough going for a while until you get your head 500 years back. I'm currently battling my way through Don Quixote in Spanish and though it's a fun and funny (not at all short) read, some of the long twisted sentences (and the old words) can be brutal. I'd bet, as above, that the English translations are breezy. I'm thinking about trying one after I finish this.
In French I'm currently working my way through Voltaire's Micromégas (which is short) but it's a hundred and some years after Rabelais and the prose is really concrete. Very short words and simple sentences. It's the story that's interesting.
Shakespeare is from around the same time period as Rabelais in OP. If you haven't read Hamlet, Othello, King Lear, As You Like It, they also take some getting used to but are worth it in the end.
I don't think that English changed as much, but I could be wrong. Shakespeare's not hard for a modern English reader, given a decent edition with vocabulary hints in the footnotes. I got through most of it as a sophomore.
Spelling fixes feels like an "allowed" cheat though IMHO. You are still with the original. Spelling wasn't very standardised and you want to get the word across. Changing word order and idioms though, then we are veering into "translation" territory.
A good middle point would be Chaucer then who had Middle English vs Beowolf''s Old and Shakespeare's Early Modern. The Canterbury Tales is frequently found in translations now, but that's also due to the lack of spelling standardization until 17th and 18th centuries in addition to just vast vocabulary differences.
There’s got to be a chronological point where older Greek gets much easier, though. Or at least, there was back when Greeks still had to work with katharevousa in school. I learned both Ancient Greek and then, though a year spent in Greece, Modern Greek. When I pick up late Hellenistic literature like Achilles Tatius, I feel like it is activating more the Modern Greek part of my brain than the 5th-century BC Athens part of my brain.
Huh. Is French from 900 years actually comprehensible to a modern speaker? English from same certainly would not be; Old English is a different language.
My native tongue is French, so in my case it was Rabelais' "Gargantua" (~1500) and "La Chanson de Roland" (~1100). Both books require some motivation to get used to their respective languages. After a while, I could read almost fluently Rabelais' prose ; it was immensely funny, coarse and impudent. Roland was harder to grasp, I had to use a specialized dictionary, but it's a concise and epic tale, and I read some verses so many times I ended learning them by rote.
Even translated, some short books from a very different culture far in the past can be challenging. For instance "Eugene Onegin" or "Gilgamesh". As a counterpoint, "The art of war" is an easy read, though written 2300 years ago in a small kingdom of China.
A last comment: the author conflate books that are difficult because of a technicality in their writing, and those that are strange in their story. The translations of Abe Kobo or Kafka I've read had nothing difficult in their words, but the surrealist plots were very unsettling. In "Pedro Páramo" the reader feels lost in a harsh world and unsure of reality. Meanwhile, Perec's "La disparition" or Becket's "Molloy" are more about style tricks.