This was a surprisingly big thing back in the early 2000s with The War Against Terror. I think that it was mostly for reasons of 'chilling effect', but the media made everyone aware that the Department of Homeland Security were paying attention to what books people took out of the library.
What was curious about this was that, at the time, there were few dangerous books in libraries. Catcher in the Rye and 1984 was about it. You wouldn't find a large print copy of Che Guevara's Guerrilla Warfare, for instance.
I disagree about how libraries minimise the risk of anyone knowing who is reading what. On the web where so much is tracked by low intelligence marketing people, there is more data than anything that anyone can deal with. In effect, nobody is able to follow you that easily, only machines, with data that humans can't make sense of.
Meanwhile, libraries have had really good IT systems for decades, with everything tracked in a meaningful form with easy lookups. These systems are state owned, therefore it is no problem for a three letter agency to get the information they want from a library.
Libraries don't tend to have consolidated, centralized IT. As a result, TLAs have to actually make subpoenas to the databanks maintained by individual, regional library groups, and The ALA offers guidelines on how to respond to those (https://www.ala.org/advocacy/privacy/lawenforcement/guidelin...).
This, of course, doesn't mean your information is irretrievable by TLAs. But the premise of "tap every library to bypass the legal protections against data harvesting" is much trickier when applied to libraries than when applied to, say, Google. They also aren't meaningfully "state-owned" any more than the local Elk's Club is state-owned; the vast majority of libraries are, at most, a county organ, and it is the particular and peculiar style of governance in the United States that when the Feds come knocking on a county's door, they can also tell them to come back with a warrant. That's if the library is actually government-affiliated at all; many are in fact private organizations that were created by wealthy donors at some point in the past (New York Public Library and the Carnegie Library System are two such examples).
Many libraries also purposefully discard circulation data so as to minimize the surface area of what can be subpoena'd. New York Public Library for example, as a matter of policy, purges the circulation data tied to a person's account soon after each loaned item is returned (https://www.nypl.org/help/about-nypl/legal-notices/privacy-p...).
Have you seen the list of books fascists want to ban? I think GP's point was exactly to emphasize that when we're talking about "dangerous books", we're talking about books that indicate you might not be a toe-lining member of The Party. We're talking about any book that any powerful person decides is some sort of threat, even if it's merely a threat to their ego.
Not dangerous at all! An analogy would be comparing a pea-shooter to an automatic rifle, or a thimble full of shandy when compared to a gallon of vodka. There is not a dangerous word in my local library!
