Public domain Greek books
Euxaristw Niovi :)
https://www.ebooks4greeks.gr/ <- This is a really good source of free books too!
https://www.smashwords.com is also a good website!
Να είστε καλά! Yes, "ebooks4greeks" is indeed a really good source! :)
Additionally, a great portal for Greek language and literature is "Η Πύλη για την ελληνική γλώσσα": https://www.greek-language.gr/greekLang/index.html.
Thank you very much :)
You are welcome! :)