I'm guessing that a book is more likely to end up in googlebooks if modern people like it. If old, it was more likely to be found by a modern person if it was popular.
It's not modern people in general, though, or even literate modern people. There's probably a bias towards geeky modern people.
There will be a bias towards books that aren't in copyright.
Quality of OCR is a factor, and I have no idea how that plays out. Might some typefaces be problematic? If so, there could be cultural/temporal effects.
Each book presumably only shows up once, which means that the effects of popularity only show up indirectly.
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