These are a few good places to start your search for primary source material:
Contains over 100 political and philosophical texts, including works by Descartes, Dewey, Foucault, Leibniz, Lombardus, Machiavelli, Maistre, Montaigne, Montesquieu, Pascal, Peirce, Plotinus, Spinoza, Suárez, Vico and Wright. Also includes some Chinese philosophy and Presocratic writings.
Artstor is a collection of nearly one million images in the areas of art, architecture, humanities, and social sciences for research and teaching purposes.
A large digitization project based out of Tufts University, this collection includes documentary papyrii, primary and secondary sources for Greek & Roman classical and Renaissance texts, 19th century American primary source documents, and texts in Old English and Old Norse for the study of the Germanic peoples, among others.
Access note: Freely available to the public.
Find these books in DePaul's Special Collections
You can search Google Books or HathiTrust to find published books online. If it was published before 1923, you have a good chance of finding the entire book online.
Immense, continuously updated digital collection providing enhanced search and availability of millions of public domain, in-copyright, and permissioned digitized books, federal government documents and serial publications in a variety of languages from sources such as Google Books, Internet Archive, Microsoft and initiatives at partner academic and research institutions.
Access note: Freely available to the public. Current DePaul faculty, staff and students get extra functionality when logged in such as full PDF downloads of public domain and permissioned works and options to create and share content collections. To log in, click the LOGIN button and select DePaul from the list.