I can't find a reference to it now, but I recently read something referencing the massive quantities of kindle data amazon give you when making a GDPR data subject access request. I think it was something like 100k rows of data for one user.

Perhaps I should do that myself.

Edit: You can request your kindle data here (UK version): https://www.amazon.co.uk/gp/privacycentral/dsar/preview.html