actually, if I agree that law enforcement can search my home, then they can search my home. I don't need a judge to tell me whether or not it is reasonable. I have that option, of course. But a library is a public institution. No one should expect that anything they do in a library would remain private. I never have. Whether the feds have a right to go into a local library and order folks around is another matter. Local cops should have been able to do it. But that's just my opinion.
I could be wrong, but I have always assumed that the records of public libraries are a matter of public record.