Website: U.S. Constitution & Related Sources: Historical Materials

Online

Constitution of the United States [National Archives]
Links to historical materials and images of the actual U.S. Constitution.
Continental Congress and the Constitutional Convention [Library of Congress]
Contains Elliot's compilation of Debates in the Several State Conventions on the Adoption of the Federal Constitution; Farrand's Records of the Constitutional Convention of 1787; and Journals of the Continental Congress.
The Federalist Papers [Library of Congress]
Arranged numerically.
The Federalist Papers [Yale's Avalon Project]
The Founders' Constitution [University of Chicago Press]
United States section of World Constitutions Illustrated [HeinOnline]
Text of Constitution and prior documents (e.g., Articles of Confederation), plus curated collection of commentaries, law journal articles, and bibliographies.

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You can search the whole collection. For example, search for gerrymander* to find nineteenth-century treatises as well as very recent commentary.

For other books, search the Library catalog by the subject Constitutional history--United States--Sources.