Google Books: https://books.google.com/
Harvard Digital Collections: https://library.harvard.edu/digital-collections
Hathitrust Digital Library: https://www.hathitrust.org/
Internet Archive: https://archive.org/
Lantern: https://lantern.mediahist.org/
Library of Congress: https://www.loc.gov/collections/
Making of America: https://quod.lib.umich.edu/m/moagrp/
New York Public Library Digital Collection: https://digitalcollections.nypl.org/
Project Gutenberg: https://www.gutenberg.org/
Smithsonian: https://www.si.edu/