Open Library — https://openlibrary.org/
Project Gutenberg — https://www.gutenberg.org/
Internet Archive — https://archive.org/
Directory of Open Access Books (DOAB) — https://www.doabooks.org/
OAPEN Library — https://www.oapen.org/
Directory of Open Access Journals (DOAJ) — https://www.doaj.org/
CORE — https://core.ac.uk/
National Digital Library of India (NDLI) — https://ndl.iitkgp.ac.in/
WikiSource — https://wikisource.org/
World Digital Library — https://www.wdl.org/

Z-Library Official Site — https://z-lib.io/