The video here has a lot of information. If you're short on time, I recommend watching the simulation of how the system would assign and deploy fighter jets and surface to air missiles to each threat, which is at about the 20 minute mark.

Here's a transcript of a panel about SAGE that occurred at the Computer History Museum in 1998. I found this passage, about why they couldn't use human spotters to warn of invasions, fascinating:

And they saw lots of things! [Laughter] They saw airplanes — many of them were civilian; they saw birds; they saw all kinds of things, and most of them they thought were Soviet bombers. I mean, this was a scary period. They would then telephone the nearest air base, which would then have to figure out if this information was worth anything. And pretty much none of it was worth anything. So, it very rapidly became obvious that despite the huge size of this program — there were 8,000 observation posts, and at the peak of the program 305,000 volunteers staffing these things 24 hours a day — the information was pretty much useless. So, commanders just ignored it. For one thing, by the time it had been verified, the bombers would already be there. So, what was the point?

The reason I'm telling you this story is that the purpose of this program was not really air defense. It was public relations. It was saying to the public, "We are doing something about this problem — we see it." At the same time, the Air Force started looking everywhere for ideas from scientists and engineers, and [referring to the slide presentation] now we're restarting, and I'm not sure what's going on.

What's the modern equivalent system that we'd use to defend ourselves against North Korean nukes? I don't know. I just hope its more than just a PR stunt.

[Computer History Museum, Wikipedia]