This is an interesting book. The authors present some things that truly raise questions. But the book falls well short of being authoritative on its central thesis. It does provide yet more proof of how opaque our federal "government" is, and regardless of what you think about UFOs that's a good "takeaway" from this book.
The authors didn't explicitly didn't claim to finally have definitive proof of anything, because there is no definitive proof (at least not available to the public). This fact is borne out in the book. The authors show time and again how investigative efforts were stymied, access was denied, and people involved in various cases were under an enforced silence. Whether the powers that be were hiding actual "space visitor" evidence or something else, we really can't know (this, however, is what the authors assert repeatedly).
Magicians use misdirection to provide us with the many illusions we see in magic shows. Misdirection has long been a high art form among the ruling class, as well. For an example of this, just consider how long organized crime has run unabated while employees of Big Oil, Big Pharma, Big Banking, and other criminal organizations pose as our "representatives" in what pretend to be two separate political parties in the US Congress. Yet no matter which one is "in power," the crime simply continues. It's all misdirection. So it's a reasonable assumption that the whole UFO thing is yet another exercise in misdirection.
My own take on the whole UFO controversy it's a way to divert attention away from the many nefarious actions undertaken by those who are supposed to govern (rather than steal). The authors did not disprove this, because their own "evidence" was mostly of the inadmissible sort. I had hoped the claims on the cover are what you find inside the book, but sadly that is not quite the case for the reason just mentioned.
The authors did provide many accounts in which people said this or that in favor of the idea that Area 51 is all about hiding evidence of a crashed space ship from some other planet. But I personally am skeptical of such accounts no matter what the issue; their lack of accuracy is a known issue among investigators. I want to see actual evidence.
I'd like to see photographs of actual artifacts, but you can't fault the authors for not having them. As demonstrated repeatedly in this book, those were not provided or permitted by those controlling Area 51. The reason might have nothing to do with hiding UFOs; it may have to do with perpetuating the controversy so as to misdirect attention.
It's not that the authors are uncredentialed crackpots, it's just that their standards of evidence are low (per the above, and I'll say more in a bit). The authors do have credentials and don't seem to be crackpots.
For example, Cary is an Air Force veteran. Not only that, he once held Top Secret clearance. To get that clearance, a person must undergo quite the rigamarole. And these authors have been investigating this topic for a very long time. They didn't just go see a movie and then decide to write a book based on a cursory view of conspiracy Websites.
In fact, the bibliography is oversized in relation to the book. Some of the references cited are from leading universities. Others are actual government documents (images of some even appear in the book). And here's were we get to the low standards part. Most of the references are dubious.
Where the authors fell down was mostly in the huge number of second-source interviews. That is, they interviewed one person about what another person (now dead) said. Those kinds of sources are especially prone to error. This pattern was pretty persistent. Rather than eyewitness interviews, we seemed to get accounts of eyewitness interviews.
This kind of "evidence" isn't permissible in court, and for good reason. Because the authors kept using it, I am inclined to discount all of the included accounts. It's a credibility issue. The authors had a duty to include predominantly unimpeachable evidence, not build a case on dubious evidence and toss in some facts. The could have used a sampling of the secondhand info as corroboration, perhaps. But it seems to be the core of their "evidence."
They also referenced a large number of UFO publications. No bias there....
And there are clues in the writing that the authors are not as careful and knowledgeable as they want to appear to be. For example, near the end of the book they talk about "Marshall Law" when it seems from the context they meant "martial law." Notice the difference in case, but also the difference in that first word. This shows a lack of understanding about a basic concept. Also near the end of the book, they "I guess...." something where they should have looked up the fact and stated it. There are other anomalies, as well. Perhaps these are the fault of a copy editor, not the authors. But I have to go by what's in the book.
There's also a fundamental head-scratcher behind the authors' basic thesis (aliens have been coming to earth, only to hide from us). If we humans found intelligent life on another planet or, even (and I know it's a remote possibility) in Washington, DC, would we not be excited and try to establish meaningful contact? Our Voyager space probe was designed partly with that goal in mind--its inscriptions are all about communicating with other life forms, not hiding from them. We have SETI and other means of trying to find and contact intelligent life on other planets. Apparently, we've given up on the idea of intelligent life in Washington, DC. But we do reach out to other, more promising places.
So it seems implausible, at best, that beings on another planet made a huge investment of resources to visit Earth and then have tried to hide the fact they are here. In fact, we have huge amounts of evidence supporting (though not conclusively) the idea that aliens were here thousands of years ago and made extensive contact (see the works of Vonnegut, among others).
But implausible is not a synonym for impossible. This book is intriguing, partly because it pushes the implausibility quite a bit closer to reality. While you're scratching your head over why aliens would come here and hide from us, you also have to scratch your head over the many issues the authors raised. Among those are the extreme efforts at secrecy and suppression. But, as I said, it could just be yet another exercise in misdirection. Given the fact that Obama's deficit spending last year alone was (according to the GAO) $6 trillion rather than the $1 trillion he stated, my vote goes to misdirection.