Like all fiction books, there are no traceable sources. All you have are words penned by a human. The author does have utmost confidence in his writing as he thankfully includes no photographs; for photographs are meaningless, especially ones that allegedly document aliens, UFOs, ghosts, spirits, ectoplasm, etc. After all, have we not all seen movie images that we know with assurity are fiction but they look so real?
That being said, if you believe that UFOs (real, as there are things seen flying which are not properly identified) are alien space craft, my words are meaningless to you and this man's tome is scripture. I cannot vouch for the quality of this work as viewed by believers; my brain is not warped in that manner.
Will a book ever convince me that alien encounters are so commonplace? I would be a fool to think so. Documented, peer-reviewed, tangible, verifible evidence will convince me in the degree that I can put my own hands on it. Admittedly, this attitude puts me in the bizarre position of not believing most everything that we all take for granted. Why should I care? Such a stance does not harm my way of life and I most assuredly do not fill my world with things non-existent, letting said things take over my life. The life of a fanatic is not for me as this book so convincingly is.