Shakespeare

Important works of English literature written before about 1920 are a little difficult. Those written before about 1820 are very difficult to read. I don't know just how difficult a challenge you want. Arthur Conan Doyle wrote 56 stories about the fictional detective, Sherlock Holmes. They're famous, entertaining, and well liked. They're out of copyright and available at Project Gutenberg. The first story collection is "The Adventures of Sherlock Holmes." They are hard to read because they are written slightly old-fashioned English, and because they have many cultural details about England in the Victorian age. They run to about twenty or thirty pages each.

"Blood Meridian" by Cormac McCarthy. I am a native speaker, and that book almost broke my brain. But it is a great story.

Well, it’s just a poem and not a full book, but Lewis Caroll’s “Jabberwocky” is famously tricky to make sense of. It is definitely available on Project Gutenberg. Good luck!