This month Popular Mechanics reported that the US government had its own flying saucer program (see Government Releases More Declassified Flying Saucer Docs). This is another set of documents from the 50s that sheds light on some of the top secret programs from the last century.
Despite the popular mythology, most scientists agree that the it is unlikely that we have been visited by aliens from space. These types of documents (and other UFO-related documents) help to show this.
If you are intersted, you may want to take a look at the library’s collection: extraterrestrials.
You may also find this interesting: 10 Alien Encounters Debunked.