The Library of Congress has published selections of its Rare Books & Special Collections for public access. This is a great resource for educators looking for primary source materials or researchers wanting access to content. The digitized selections include letters, images, and other important documents that are housed by the special collections division of the Library of Congress.
For example, see the “Account of Louisiana” put forward to Congress by President Thomas Jefferson, the early drafts of the American Constitution, De revolutionibus orbium caelestium by Copernicus, and more. The collection is free and available to the public.