[dredmorbius]

I'm a fan of the Samizdat sources (you could add ZLibrary as well).

Archive.org's holdings are truly amazing for older works. Newer, in-copyright works can be checked out, though I find the e-reader software, which works well on desktop, is poorly-suited to tablets.

Other sources of legal works include:

- Project Gutenberg: https://gutenberg.org/ (60k books)

- Standard Ebooks: very-well formatted quality ebook, largely public domain https://standardebooks.org/

- Wikisource: https://en.wikisource.org/ Public-domain works, downloadable in ePub and other formats.

There are numerous other smaller collections focusing on specific topics which may also be useful. Searching for "filetype:pdf", "filetype:epub", or "filetype:djvu" may find ebook formats elsewhere.