Interestingly when I hear the author describe different kinds of "difficulty" I find myself thinking that some of those don't really seem like difficulty. For something like a book, I think the main thing that makes it seem difficult is when there are parts that are not fun to read (or maybe moments where you question the point of the entire read because you pause to go "what the heck is even going on here"). If something's fun it often doesn't really feel difficult.
In particular various kinds of formal experimentation can be quite fun and I wouldn't perceive them as difficult at all (especially in shorter form). I read a short story in multiple-choice form years ago and loved it. I've read most of Milorad Pavic's books, all of which have unusual formal structures (e.g., Dictionary of the Khazars has the nonlinear structure of a cross-referenced dictionary or encyclopedia). Some were a bit baffling (they also can fall into the "confusing events, surrealist dream logic, and elliptical plots" category) but I didn't find them difficult exactly.
One of the more common practical difficulties that can add friction to a book is a complex storyline with many characters who can be hard to keep track of. But this won't make the book as a whole feel super difficult as long as the content is worth it. War and Peace is a classic example of this, with multiple intersecting storylines and a large cast of characters. A Suitable Boy is a modern example in a similar vein.
But I think a lot of times when people say a book is difficult they just mean either "gosh I actually kind of have to pay attention to this" or "this is really long". To me those things actually are positive qualities if the content of the book is good, since they just make it richer. My favorite novel is In Search of Lost Time, which is one of the quintessential "difficult" books, but if you get into its rhythm, most of it is blissfully engrossing.
In particular various kinds of formal experimentation can be quite fun and I wouldn't perceive them as difficult at all (especially in shorter form). I read a short story in multiple-choice form years ago and loved it. I've read most of Milorad Pavic's books, all of which have unusual formal structures (e.g., Dictionary of the Khazars has the nonlinear structure of a cross-referenced dictionary or encyclopedia). Some were a bit baffling (they also can fall into the "confusing events, surrealist dream logic, and elliptical plots" category) but I didn't find them difficult exactly.
One of the more common practical difficulties that can add friction to a book is a complex storyline with many characters who can be hard to keep track of. But this won't make the book as a whole feel super difficult as long as the content is worth it. War and Peace is a classic example of this, with multiple intersecting storylines and a large cast of characters. A Suitable Boy is a modern example in a similar vein.
But I think a lot of times when people say a book is difficult they just mean either "gosh I actually kind of have to pay attention to this" or "this is really long". To me those things actually are positive qualities if the content of the book is good, since they just make it richer. My favorite novel is In Search of Lost Time, which is one of the quintessential "difficult" books, but if you get into its rhythm, most of it is blissfully engrossing.