Hope you aren't an anomaly, have interests / read material that could be used against you or are similar to whatever society thinks the boogyman du jour reads.
With the restrictions the Wassenaar Arrangement can set up, you'd be surprised how quickly many of your mundane professional interests will put you on the radar.
Section 215 of the Patriot Act expired in May 2015, but there is great interest in reauthorizing the rights granted to the federal government from that section. For 14 years, the government had the right to track your library usage and issue gag orders to librarians involved.
To say they destroy those records would be an oversimplification. Fighting the government is hard when you're issued a warrant from a secret court and are facing serious prison time for not obeying it.
Libraries don't keep records? That's how the killer was caught in the film Seven
If you knew the material you were interested, say "alternative music"[0] might put you on a list, would you check the book out or try to stay off the library's inventory system?
With electronic books, browsing is checking out is an expressed interest in what can make you seem like a terrorist/spy/hacker/who knows to systems that are used to identify enemies of the state.
[0] http://www.theguardian.com/australia-news/2015/sep/25/radica...