Above Top Secret: The Worldwide U.F.O. Cover-Up
I can't say that I agree with everything Good proposes in this book, but he does raise some interesting possibilities about UFOs. Good is not claiming that UFOs are extraterrestrial, but simply that they do exist and that the governments of the world have been hiding evidence of this fact.  His study is exhaustive but would still be worth reading just to examine the facsimile copies of confidential and secret documents from the U.S., Canada, and Europe, as well as the controversial MJ-12 documents.