1. https://www.gutenberg.org/
2. https://openlibrary.org/
3. https://www.planetebook.com/
4. https://archive.org/
5. https://www.smashwords.com/
6. https://books.google.com/
7. https://www.freetechbooks.com/
8. https://www.getfreebooks.com/
9. https://www.openculture.com/free_ebooks
10. https://www.goodreads.com/
11. https://www.oreilly.com/ (trial)
12. https://annas-archive.org/
13. https://pdfcoffee.com/
14. https://singlelogin.re/
15. https://www.ereaderiq.com/freebies/
16. https://www.bookbub.com/ebook-deals/free-ebooks
17. https://digilibraries.com/
18. https://www.overdrive.com/
19. https://manybooks.net/

there’s so many others and of course torrents