I've been reading the Delta Green campaign book and I've got to say that other than a fair few typos in the hard cover edition that I've got, it's a pretty solid book. Nice lay out, absolutely beautiful and evocative pictures, and some really interesting text that tells a story even as it delivers the detail. That latter point is very important, I find. I've read books where the text is just text and, whether fluff (world building, etc.) or crunch (mechanics), it's incredibly dull and doesn't have to be. It's important that a game book tells dozens of little stories there as it is, in the end, a story reference.
I don't mean that each section starts with 'Once upon a time' but that there's a sense of pacing to the revelations, perhaps an anecdotal feel to some of the facts like a friend were telling them to you, and certainly a sense of bias that runs throughout it to give you a sense of the mood and atmosphere of the game. The government may be deluded in Delta Green but it is out-matched, out-paced, and involved in conspiracies, or so the bias in Delta Green goes.
So what is Delta Green about? Well it's Call of Cthulhu updated for an X-Files vibe and set in the modern era. I haven't read it all but already I feel that it's a great resource for that type of setting just because the text itself is so interesting it's easy to get inspired. It also has a really good timeline for FBI and CIA actions (especially CIA actions) and government conspiracies, real or fake, so even if you weren't running Call of Cthulhu you could read it and get ideas. This is especially the case since there is a focus on Majestic-12, Karotechia, and the alien race, all of which could be transplanted from a literal Call of Cthulhu world quite well.
Would I play it myself? Well I love conspiracies and investigations, so absolutely. I'd be a little sad if it occurred now that I've read the source book but that's just what happens sometimes. Since I run so many games and take advantage of a lot of source material, it is quite hard to get me to play something I haven't read something about. Not impossible, but difficult.
Would I run it? Possibly. It would depend on the player base I had at the time. I'd probably have to put it in World of Darkness rules simply because some of my players aren't keen on learning new systems although BRP is an easy one to learn. I've never run that kind of modern science fiction before but I'm sure that after watching a couple seasons of X-Files I'd be all set to jump in the deep end and get something cranking.
All in all, a good book. No wonder its still in vogue.