This list could go on forever and ever, depending on how far you want to go with the term "foundational".
Other works that shouldn't be dismissed:
The Magna Carta of 1215
The Petition of Right 1628
The English Bill of Rights of 1689
Locke's Second Treatise on Government (1689)
Declaration and Resolves of the First Continental Congress (1774)
Petition to the King (1774)
The Olive Branch Petition (1775)
Henry's "Give me Liberty or give me Death" speech (1775)
Lee Resolution (1776)
The Declaration of Independence (1776)
The Articles of Confederation (1776)
The US Constitution (1787)
The US Bill of Rights (1789)
The Northwest Ordinance of 1789
The Wealth of Nations was published in 1776, so it had no impact on the foundation of the US. Democracy in America was published in the 1830s and really speaks more to the United States in that period, but is certainly an excellent source.
Two others I hear about: de Tocqueville's "Democrary in America," and Adam Smith's "The Weath of Nations."
Any value of either of those?
Just don't fall into the trap of thinking that the "Founding Fathers" set in place a static government, and that their interpretation is the best / only valid one. The Civil War amendments pretty thoroughly changed fundamental interpretations of the Constitution far beyond allowing black people to vote, just for one clear example.
So a syllabus like this one can give you plenty of important readings:
https://ocw.mit.edu/courses/history/21h-224-law-and-society-in-us-history-spring-2003/readings/
There are two major subcategories of documents that answer your question. The first contains documents specific to the American style of democracy (e.g. Common Sense, Declaration of Independence, Bill of Rights). The second category would be documents, primarily written during the Enlightenment in Europe, that provide the theoretical framework for democracy/republicanism (i.e. documents that discuss ideas of right to rule, social contract, state of nature, separation of powers).
In the first group, I'd only have one document to add to what others have posted with:
-Virginia Statute on Religious Freedom [link] (https://founders.archives.gov/documents/Jefferson/01-02-02-0132-0004-0082)
The above document is especially interesting when compared to the later Bill of Rights
For the second category, the following documents are absolutely essential to understanding American government (I'm only mentioning ones that other posters have not included):
-The Social Contract by Rousseau
-The Leviathan by Hobbes -On Crime and Punishment by Beccaria