I'm not sure that there's any restriction or erosion of rights going on here. The public library is a service that the county (or possibly city or state) chooses to provide, and if they want to keep a list of what you check out, that's there business. I don't think that there's any useful information to be gathered, though. Anyone who checks out books on chemical synthesis or airline piloting when planning a terrorist attack probably isn't going to be succesful in the first place.