There's a good list of sources which have influenced various members of the Hall at the Common Ground: Sources post. Here, I thought it might be useful to link the shorter ones that could be read relatively quickly. The longest is "One Day in the Life of Ivan Denisovich," a novella that might take a couple of sittings. I have included links to the Wikipedia articles on these documents as a starting point for understanding their context, history, etc.
Feel free to add more short sources in the comments, or to give related sources and links (e.g., websites or books that explain or interpret these sources).
The Magna Carta (This is the National Archives page on the document. Here is the text.) (Wikipedia article)
The Declaration of Arbroath (This is the National Archives of Scotland page on it. They offer a PDF with the original Latin and translation in English.) (Wikipedia article)
The Declaration of Independence (Wikipedia article)
The Constitution and Bill of Rights (Wikipedia article)
NB: The Declaration of Independence, Constitution, and Bill of Rights pages are part of the National Archives's Charters of Freedom website, which has a number of pages which explore the history and impact of these documents.
"Harrison Bergeron", Kurt Vonnegut's short story about the push for complete equality (or, depending on your interpretation, his sarcastic attack on those worried about the push for complete equality) (Wikipedia article)
"One Day in the Life of Ivan Denisovich", Alexander Solzhenitsyn's novella about life in the Soviet gulags (Wikipedia article)