They're basically rebuilding Google books, and keeping it private. That's privatizing all of humanity's knowledge. Then from what I see some other missing books are just rephrasings of other books, which can already be done with synthetic data using another LLM to generate it. I would assume a lot of those books are school text books as well, which are almost never digitized and are rephrasings of university text books. Maybe they are not able to generate synthetic data of sufficient quality and are resorting to human books that rephrase information instead