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Project Gutenberg, a volunteer project founded in 1971 by Michael S. Hart, was the first digital library. 

Later leapfrogged by much better financed non-profit or government-sponsored or commercial digitization projects, in 2018 Project Gutenberg offered 56,000 free e-books. 

The history of this pioneering project is enumerated here:

http://www.gutenberg.org/wiki/Gutenberg:About