Library records were not necessarily all that private.<p>When I was a kid&#x2F;teen, each book in the county library had a card in a pocket glued or taped to the inside front cover of the book. Your library card had a metal plate on it embossed with your library card number. To check out a book you took it to the checkout counter, they took your library card and the card from the book, and used a machine that printed your library card number and the due date on that card from the book [1]. They kept that card, and put a card in the book that told you the due date.<p>When you returned the book, they would put that original card back in the pocket and return the book the the stacks.<p>Note that if you could obtain my library card number, then you could find out if I had checked out a particular book by simply finding that book on the shelves and looking at the checkout card to see if my library card number was printed on it. You could even tell when I checked it out because the due date would be listed. The card had enough room to store records from a couple dozen checkouts.<p>I think that this didn&#x27;t bother people because it took some work to get the information. First, you need the target&#x27;s library card number. One way you could get that would be to have someone watch me when I come out of the library and try to get a good enough look to identify a book I&#x27;ve checked out. Then wait until I return the book and it is back on the shelves, and you can get my number from the card.<p>Once you&#x27;ve got the card number, you now have to start looking in books for it. If you are trying to see if I&#x27;m reading books on a particular subject, you might reasonably look at all books on that subject, but if you are trying to get a general picture of all my reading that is going to be a big hassle.<p>A lot of things were like this--the data was not particularly protected or kept private, but because getting it required going to some particular place and trolling through printed records generally only people who had a really good reason to want it would go to the effort.<p>[1] Similar to the way credit cards worked back before electronic card readers.