What an undertaking this must have been. Having personally met a number of the witnesses who testify in Dr. Greer's book, I find them to be convincing.
The evidence revealing the huge extent of covert development programs to back-engineer ET technology consuming trillions of dollars channeled through black budgets is mind-boggling. But there is enough evidence that this has been happening for decades to convince any but the most self-deluded skeptic. Not only from witnesses who testify on oath, but actual docs released theough FOI both in the US and elsewhere.
I know Greer has a reputation of having a hugely inflated ego, and creating what amounts to an almost religiously zealous following who are fleeced of their hard-earned money at every opportunity. That's as may be, I don't know. But 'Disclosure' is certainly a great book and if you don't know how many hundreds of ex-military and highly-placed government people have gone on the record under their own names to testify that the ET issue is real and that the governments of the world are hiding the truth from the people, then this is a good place to start.
It's long, though. At 570 pages you're not going to get through it in an evening. But it's also engrossing, and improbably, a page-turner.