• The Feynman Lectures on Physics (English)
  https://www.feynmanlectures.caltech.edu/info/
• Global Grey (English)
  https://www.globalgreyebooks.com/authors.html
• Motion Mountain (English)
  http://www.motionmountain.net
• The On-Line Books Page (English)
  http://digital.library.upenn.edu/books
• O'Reilly Open Books (English)
  https://openlibrary.org/publishers/O%27Reilly (History)
• Perseus Digital Library (Many Languages)
  http://www.perseus.tufts.edu/hopper
• Retro-Bibliothek (German)
  http://www.retrobibliothek.de/retrobib/suche.html
• Rheinwerk Openbook (German)
  https://www.rheinwerk-verlag.de/openbook/
• Shakespear's Complete Works (English)
  http://shakespeare.mit.edu/works.html