NICAP (National Investigations Committee on Aerial Phenomena), have placed authorized full length versions of books, online for everyone to read for free. They are not up to date books, but are worth reading (admitedly i have only read a few, but glancing through the rest, they look interesting . The link to them is here ) .
The books are:
“Alfred Loedding and the great flying saucer wave of 1947” (by Michael hall and Wendy connors)
“The flying saucers are real” (by Donald Keyhoe ) 1950
“Symposium on Unidentified flying objects” second session 1968 (by various)
“Regional encounters : The FC files acentury of ufo sightings and close encounters in the midwest” (director : francis ridge) 1994
“Report on the ufo wave of 1947” (ted bloecher) 1967
“Flying saucers : fact or fiction” (Max Miller) 1957, i think
“The report on Unidentified Flying objects” (Edward Ruppelt) 1956?
“The ufo evidence” 1965 (published by NICAP)
“Ufos: a new look” a special report by NICAP (1969)
“ufology” by James McCampbell (/)
“Inside saucer post 3-0 blue” a summary report by Leonard Stringfield (1957)