> I wouldn't be surprised if even proprietary content like the books themselves found their way into the training data

No need for surprises! It is publicly known that the corpus of 'shadow libraries' such as Library Genesis and Anna's Archive were specifically and manually requested by at least NVIDIA for their training data [1], used by Google in their training [2], downloaded by Meta employees [3] etc.

[1] https://news.ycombinator.com/item?id=46572846

[2] https://www.theguardian.com/technology/2023/apr/20/fresh-con...

[3] https://www.theverge.com/2023/7/9/23788741/sarah-silverman-o...