Operation Trojan Horse is essential for any student of the occult. Mr Keel's brilliant thesis on the "ultraterrestrial" hypothesis needs to be read . . . albeit, with the proverbial "grain of salt."
You see, shortly after the 1970 version of this book was published, Mr Keel was not only "contacted" by the subjects of his research -- they refused to leave him alone! As a result he was driven insane. This is most clearly evident in his later work, "The Eighth Tower." A few of Keel's other works ("This Haunted Planet" and "The Mothman Prophesies") are noteworthy, but not in the same class as OTH.
The opinion of the ultraterrestrials regarding this book seems to have been, "He wrote WHAT??? Oh, no -- can't have that! Let us put him at the top of our list of folks who need to be f**ked with. . ." The first edition almost did not see print, and a 2nd edition was never published until this revised edition came out in the late 1990s. Meanwhile, between Keel stating things like, "God is insane, and an ancient computer keeps sending BEAMS INTO MY HEAD!" (paraphrased from "The Eighth Tower"), and allegations of ultraterrestrial Dopplegangers impersonating him across the land -- well, let us say that his (ahem) "credibility" has suffered considerably.
Regardless of Keel's mental illness (from which he eventually seemed to recover), sloppy research, and bizarre speculations; OTH stands out as perhaps the best thing ever written about multi-dimensional entities. Keel spends a lot of time discussing the nature of the electromagnetic spectrum, and goes in depth regarding little known facts common to many witness accounts (things which were left out of official reports, as they contradicted the preconceptions of "scientifically biased" investigators). Keel's investigation shows that this sort of thing has been going on throughout recorded history. Starting with ancient mythology, and progressing onto folk legends, then delving into old reports of "airships," "ghost planes," "foo fighters," and modern "black helicopters," Keel presents a convincing argument that these are not the result of human conspirators, but rather a multi-dimensional intelligence which can assume any guise it wishes.
Perhaps the most important point that Keel makes is that these intelligences ARE NOT TO BE TRUSTED. Indeed, he states that the many reports of "aliens" observed performing nonsensical actions (making "repairs," collecting soil samples, performing medical examinations, and delivering lectures about the dangers of atomic energy to inbred hillbillies) are nothing more than a charade intended to make us believe that they are something other than what they are. Keel believes that the only reason for such a ruse would be hostile intent. He then expands this argument by referring to the hundreds of "contactees" (as well as spirit mediums & New Age "channellers") who have been given a series of valid prophesies -- which then compel them to either go on a wild goosechase, make public announcements of impending doom, or even form destructive cults.
But consider this . . . what if it isn't "hostile intent" at all? What if the ultraterrestrials are just kicking back, drinking some beers, and one of 'em goes, "Hey -- you know what would REALLY mess with their minds?" Perhaps it is nothing more than a warped sense of humor!