There's cetainly a question about whether the books are difficult or just more work than normal. I wondered what books you found difficult that weren't on the lists.
I prepared for Tolstoy and was pleasantly surprised at how good it was despite the length. I didn't get on with Naked Lunch, but I found The Waste Land intriguing, and have regularly returned to it over a number of years. I think I realised that it was an influential and important poem, without fully undrstanding much of it when I first read it.
I read To The Lighthouse whilst doing my A levels, and it really had a positive effect upon me, though I had, at that time, nowhere to channel what it did. I think that reflects the limitations of the Eng |Lit course at that time. I may well have followed it up with more relevant reading if I had understood its import.
I also enjoyed The Gulag Archipelago, though I know, due to my ignorence of much of Russian history at that time, that I didn't get as much out of it as I might have. It is one of the few books I intend to re-read.
I read Sartre's Nausea and Iron in the Soul when I was in my twenties, and I felt that I didn't fully understand the implications of it due to my ignorence of the philosophy. These were difficult books for me, as I was aware that I wasn't getting them.