[JLO64]

As others here have mentioned, https://standardebooks.org/ is excellent and my understanding is that they use Gutenberg books as a source for theirs but done up much nicer.

[everybodyknows]

You can contribute to Standard Ebooks by finding OCR errors, then pushing your fixes to https://github.com/standardebooks

[dempedempe]

Source can be anything with the original text, but, more often than not, ends up being PG.