One Day in the Life of Ivan Denisovich by Aleksandr Solzhenitsyn
This is one of those novels where the historical significance dwarfs any discussion of its literary merit, which is still considerable. It's a slim, effective story that hits pretty much all the right notes within its premise of giving us a single day in this guy's life. The artistic nerve required to even publish it just adds to it. Like a lot of stories, I suppose it has lost a bit of its impact with time, since its description of the gulag doesn't have the same novelty it did in 1962.