The hardest book I never finished? Gravity's Rainbow. I have no idea why Pynchon decided to write the book that way - his other books aren't like that. With GR, virtually every sentence is incredibly dense, and it isn't helped by the fact that he loves to use archaic words that send me scrambling for my dictionary at times. Plus, the story makes very little sense, it jumps all over the place and it's purposefully bizarre. It's unique, I'll give it that, but little else recommends it. Which is a shame because Pynchon is a good writer.
The Hardest book for me to get through, emotionally, was "Humanity" by Jonathan Glover. It's a history book... but it's a history book concerning humanity's worst actions. It covers all the worst atrocities, how they happened and how otherwise normal people could be convinced to carry out atrocities against their fellow human beings. The book spares nothing - The Holocaust, Stalin's GULAG camps, the Khmer Rouge's "Year Zero" revolution, My Lai, Rwanda, Battle of the Somme, The Serbian-Bosnian wars, the treatment of Allied Prisoners in Japanese Camps, the Rape of Nanjing, the Atomic Bombings of Hiroshima and Nagasaki, the treatment of Russian POWs during WWII, and the treatment of German Civilians at the end of WWII. I was left pretty badly shaken after reading that. And I was about 13 years old at the time. It's still one of the most important books I've ever read, but you'll gain a new insight into how evil and how good humans can be. At the end, it does offer up a bit of hope by providing examples of people who, despite all their brainwashing, tried to help the innocent. But it's a mostly horrific glimpse into what people, ALL people, are capable of.
The Most disgusting book I've read has got to be "Dice Man". I know, a lot of people like that book. I didn't - the main character was a psychopath. Being a "rebel" shouldn't give you license to hurt others.