Kurt Gödel's Letter to John von Neumann (1956) [pdf]
The story of Von Neumann's death is actually quite interesting. The government kept a guard by his death bed to be sure he didn't divulge any state secrets while on mind numbing pain medications. It is well documented that, after Von Neumann learned of his terminal pancreatic cancer, he became horrified of death and would cry out at night, even resorting to last-minute conversations with a priest on his deathbed (though he was an atheist throughout his life). His mother died during his illness and his family tried to hide it from him. But he eventually figured it out and took a turn for the worst. It's all very sad in the same way it would be for anyone, but in Von Neumann's case it makes one wonder what else he would've accomplished in the next two or three decades of life.