Major Keyhoe has written a few earlier books on this subject that are required reading for many in ufology. He always tells is like it is, and has many facts and actual cases to back up what he puts in his book. He states the nuts and bolts ET hypothesis and has kept his files intact.
The only problem about this book is he does not cover much about abductions or crashes, which were considered crackpot for many of the researchers back then. (He does give some credence to the Hill case, but that is the only one he really mentions in depth.)
Written in 1973, there are many very good cases he goes over, mostly known in ufology circles. There are several good ones that do not get any publicity, and it is good to read about those cases.
Available at used bookstores, libraries or even some garage sales, if one sees this book they should pick it up.