This is why I keep "pirating" and hoarding books. I know it's bad for the common good and bad for the authors but I feel paranoid about corporations and national security agencies collecting and later, perhaps, analyzing what I read, including how long I linger over a page, what I underline, when I take screenshot excerpts, etc.
I started innocently enough, by scanning all my physical books (and downloading the epubs or pirated pdfs of books I already paid for in physical form whenever available) just to lighten my steps, so to speak. Then I came upon a passage in 1984 where books were altered, entire passages excised and rewritten to suit whatever powers that be. It was completely unrealistic when 1984 was written, of course, but in 2013 (when I read it), this hit a little bit too close to home. With all our devices constantly itching to phone the mothership, this is not a fantasy anymore but a possibility. All that stands between Orwell's dark vision and reality is, in fact, political will and our collective memory of times when this was not possible. Perhaps future generations who do not remember physical books will more easily acquiesce to such intrusions for the sake of "mental hygiene" and, you know, the children and stuff.
Anyhow, today I look at a 16K+ Bibdesk database which, of course, includes the files/ebooks on my airgapped machine and feel curiously satisfied. Not in a lifetime will I be able to read all this but I know that I could spend my remaining days reading freely, no matter the external circumstances. Freedom of thought! (Also, a very neat and nifty system for me personally I admit... :)