Glory is about two scientists from the intergalactic Amalgam culture who arrive at a world in the throes of something similar to our Cold War phase coupled with expansionist thinking. Meanwhile, the artifacts of an ancient race on the this world that never went space-faring, but in looking inward did highly advanced mathematics before they went extinct (a unified field theory even the brightest minds of the Amalgam haven’t figured out yet), is under threat of destruction from the realpolitik and sentiment of the day.
In a very simplified way the story contracts two opposed viewpoints, cultures that want to spread and multiply for the sake of it, and cultures that seek knowledge just for the sake of it. And while the Amalgam is countless levels more powerful than the local culture the two explorers visit, one of them realizes what gaining something like the holy grail of knowledge could do to the intergalactic culture. After all, the ancient culture vanished once it found its ultimate mathematical proof.
Egan’s writing has two very strong characteristics, but one of them, the ultra hard SFnal aspect, gets mentioned more often than the other. Egan is one of the few science fiction writers who creates optimistic futures. This idealistic notion that human or intelligent and consciousness life can better itself, not just technologically but also morally, is an equally strong part of his fiction as his hard sf notions.
But with paradises or utopian societies (real ones) often comes the notion that they will inevitably fade away, because people who have left behind wars, diseases and even overcome death, will inevitably decline because of a lack of challenge. What’s left is the search for knowledge, but what if even that is finite? The story doesn’t give a clear answer, no real solution.
What it gives is hope, hope that even when the external source of knowledge seems to have dried up, when it looks like science has ended, the internal variety of the Amalgam, the diversity of an galactic civilization with countless sentient races, will be enough to carry through until the moment when new directions for the search for knowledge have opened.
If you want an interesting counterpoint to this story, read Peter Watts story Ambassador, which is IMHO thematically related, even if it goes at the whole theme from a completely different direction and takes a completely different position. But then, Peter Watts stories are rarely outbursts of happiness.