In The Atlantic, Noah Berlatsky says that a book's "difficulty" is subjective. The opening might grab you--
What's the most difficult book you've ever read? For me, at least within recent memory, there's no question—the book that was hardest for me to slog through, the book that I would have put down if I didn't have to read it for work, was E.L. James's Fifty Shades of Grey.
Yeah, well . . . I don't think I'm buying this definition of "difficult." Among people who sit around writing essays about difficult books--by which they mean, right from the start, something that has some objective claim to merit, at least to others in that group--they are leaving commercial entertainment out of the discussion. To this group, that other kind of writing is "bad." And when they say reading that sort of "bad" entertainment is "difficult," they don't mean it's difficult for them to understand, but difficult to stomach.
That's my riff, in any case, on the first paragraph of Berlatsky's essay. It's an essay that I liked a lot, despite my questions about whether he's setting up a straw man by defining "difficulty" to mean something other than it normally does in discussions about books.
Full essay: "Readability Is a Myth," by Noah Berlatsky.