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/