I'm in a humorous mood at the moment so I'm going to have to go with A Confederacy of Dunces [http://en.wikipedia.org/wiki/A_Confederacy_of_Dunces] by John Kennedy Toole. It's quite simply the funniest book ever written and I'm rather disappointed no one else has mentioned it. Other books I might choose depending on my mood include The Two Towers, 1984, Through the Looking Glass, Breaking Dawn, Speaker for the Dead and The Count of Monte Cristo.