I'm curious if this is still a thing. Libraries in my county have moved to a completely online system for checking out books, renewing, etc. The system regularly recommends books to me 'based on previous borrows', which indicates they are tracking/storing information about what I have borrowed in the past. The next question is, who has access to this information? My guess in the library system here does not have a strong infosec department, nor a strong legal department to resist subpoenas/police shenanigans.