The Salem Witch Trials, the Declaration of Independence, the Federalist Papers, or Andrew Jackson's bank veto message will not make for a very original write-up.
There are all kinds of potential documents related to business, politics, the emergence of science, medicine, religion, technology, sports, family, and just about anything else you can imagine.
The main restrictions are that the document has to come from North America, it has to have been created before the year 1877, and it cannot be a document that is already well-known or that has been written about extensively by historians.
Was the author a direct observer of the event/issue [if this is pertinent] or was the information obtained second-hand? What light does is shed on the society/events/issues described? Are there allusions made by the author that need to be explained? Believability of the document: Given the external analysis and the content of the document, how credible is the information?
Had the author any personal involvement in the events/issues described [if pertinent]? Is it contemporary to the events/issues it describes? Do not only summarize but analyze the document as well: What does the author mean?