The UFO Encyclopedia, Volume 1: UFOs in the 1980s.

Detroit, MI: Apogee Books, 1990.
234pp.
ISBN 1-55888-301-0.

This encyclopedia is mostly the author's tour de force, but still delivers valuable information not easily available elsewhere.
Libraries should have this.

The UFO Encyclopedia, Volume 2: The Emergence of a Phenomenon: UFOs from the Beginning through 1959.

Detroit, MI: Omnigraphics, 1992. 433pp.
ISBN 1-55888-301-0.

The second volume of this fine work is must reading for those desiring real understanding of UFOs.