This link takes you to a list of Banned books accessible online via such sites as the Gutenberg Project--edited by John Mark Ockerbloom of UPenn.
Gutenberg Project - Free eBooks
Librivox - Free public domain audio books
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Banned Books material in DPLA (Prepared list of search results)
The Internet Archive is a 501(c)(3) non-profit that was founded to build an Internet library. Its purposes include offering permanent access for researchers, historians, scholars, people with disabilities, and the general public to historical collections that exist in digital format.