This question has me confused. Does it mean actually hard to read as in 'hard to follow', or does it mean emotionally hard to read? I skipped ahead a few questions and saw that there is a question coming up dealing with emotional difficulty so I'm going to go with my first assumption. For that question, there can be no other answer than Ulysses by James Joyce.
I can't begin to tell you how many times I have started reading this book and set it aside never to return. One of the problems is that it ones of those books, like The Tin Drum, which has so many references to other literary works and ideas that it really should be read as part of a class so you can actually understand half of what is being said (without words in many cases) on the page.
About a year ago, I broke down and bought myself an annotated version when it was on sale but never quite got around to cracking it open. Some day I will give that sucker a go and if I still can't finish it... well, I'll buy the Cliff Notes and start lying.
"I've put in so many enigmas and puzzles that it will keep the professors busy for centuries
arguing over what I meant, and that's the only way of insuring one's immortality."
~James Joyce, on Ulysses