Now we have a whole 'new' area to 'investigate'.
Under federal law prior to the Patriot Act, any law enforcement agent could get a warrant to search a particular person's library records if a person could be identified, or even the entire library record if a particular person was not identified, upon a showing of probable cause. For example, you find a copy of a book at a crime scene and it's from a library. If the library didn't open up the records, any judge would issue a warrant to find out who checked out that book. Similarly, it's determined that someone used a library computer to access classified information, but the library won't say who was using the computer. Any judge would again allow a search of the user database as there was probable cause that someone committed a crime and there was evidence of the offender's identity in that database. All that was going on prior to the Patriot Act, and is perfectly consistent with the 4th Amendment.
These searches could also cover a vast area of turf. For example, if you are suspected of conspiracy to commit terrorism, a proper search could include books on: political theory, theology, explosives, weapons, infrastructure, etc. Anything that justified your action, formed the basis for your intent, would allow you to further the conspiracy would fall within the type of evidence which could be examined. It's just a question of explaining to the judge why the agent thinks the material is relevant.
I agree that the idea of an agent being able to access records may have a chilling effect on some. But I think it's important to be able to identify our target. The issue is overreaching by the gov't, and the Patriot Act is a symptom of that, and focussing our ire on it runs the risk of masking what should be the true target.
)