Some useful books and monographs on the UFO phenomenon are available online:
Richard Hall, ed., The UFO Evidence (National Investigations Committee on Aerial Phenomena, 1964)
Leon Davidson, Flying Saucers: An Analysis of the Air Force Project Blue Book Special Report No. 14 (The Author, 3rd ed., July 1966)
A. Schopick, A Study of Secondary Effects (The Author, Summer 1966), an unpublished manuscript describing electromagnetic (EM) effect UFO cases
Ted Bloecher, Report on the UFO Wave of 1947 (The Author, 1967). Other classic UFO books are available on the NICAP site.
J. Allen Hynek, The UFO Experience: A Scientific Inquiry (Corgi, UK ed., 1974)
J. Allen Hynek, The Hynek UFO Report (Sphere, UK ed., 1978)
George M. Eberhart, UFOs and the Extraterrestrial Contact Movement: A Bibliography (Scarecrow, 1986), volume 1, volume 2
Willy Smith, On Pilots and UFOs (UNICAT Project, 1997)
Thomas E. Bullard, The Sympathetic Ear: Investigators as Variables in UFO Abduction Reports (Fund for UFO Research, 1995)