Largest secret library

The library of the Central Intelligence Agency, located at CIA headquarters in Langley, Virginia, USA, holds some 125,000 books, as well as numerous periodicals and reports. Most of the material in the library is not classified, but the details of its holdings remain an official secret, as just the catalogue data could give information about the agency's interests. Library cards are only granted to US government employees with the relevant security clearances.

Sub categories within the CIA collection that have been disclosed include the "Historical Intelligence Collection" and "The Whaley Denial and Deception Collection", which apparently contains books on confidence tricks, secret communication methods and sleight of hand.

The Vatican is often described as having a "secret" library, whose contents are the subject of some wild speculation, but this is not the case. The Bibliotheca Apostolica Vaticana is a real place, and with more than 1 million books, it's a significant collection, but it isn't secret. The catalogue can be searched online and many of the older books have now been digitized. Access to the physical collection is limited, but this is due to the need to fairly share access to a globally important scholarly resource. Rumours of a "secret library" are likely inspired by the Archivo Apostolico Vaticano (the Vatican Apostolic Archive), which is a collection of papers, correspondence and other materials relating to the running of the Catholic Church over the centuries. Access to this facility, which does not hold any printed books, is much more tightly restricted and large sections of the collection are off-limits to the outsiders.