These are a few documents that reflect the attitudes of the US and other governments regarding the UFO phenomenon.

Top-Secret 1949 Document, by Jan Aldrich
                Report by the USAF Director of Intelligence to the Joint Intelligence Committee on Unidentified Aerial Objects, April 27, 1949

Air Force Advisory Panel Briefing by the Condon Committee
                January 12, 1967

Observations of Anomalous Atmospheric Phenomena in the USSR: Statistical Analysis, by Lev Gindilis, D. A. Menkov, and I. G. Petrovskaya
                USSR Academy of Sciences, Institute of Space Research, Report (1979)

The CIA and the U-2 Program, 1954–1974, by Gregory W. Pedlow and Donald E. Welzenbach
                CIA Center for the Study of Intelligence (1998)

The French COMETA Report on UFOs and Defense, by Gildas Bourdais
                A summary (July 1999)