The most important document that inspires me in my career is De Profundis, by Oscar Wilde.
Written to his friend, Lord Albert Douglas, while Wilde was in prison, this is an essay of over 50,000 words, written over the winter months of 1897. While it may not be a document that changed history, it was created while Wilde was jailed for public indecency. The indecency? Having a male lover in the year 1897.
Beyond the poignant essence of this massive letter, it resonates with me for the precise, clear writing under adverse conditions. You see, Wilde was not only in prison, but he was allowed only one sheet of blue onion-skin paper per day. At day's end he had to hand the day's page back to the prison guard in order to receive another sheet of paper for the next day's writing. And as you can see, he used every inch of the paper.
I had the pleasure of viewing this document on display at The British Library in London. It was a marvelous, moving experience for me to see it firsthand. Imagine the mental clarity needed to write under such circumstances, and receive inspiration from the conviction and focus of this writer.
As it exists now, available at Project Gutenberg.org and on Wikipedia, the enormous letter remains as it was written, and was apparently never revised when Wilde was allowed to take it with him upon release from prison.