This is a follow-up to The Star Dwellers. Blish examines possible pitfalls for societies with high-energy economies as his protagonists venture into the heart of the galaxy on a mission to force admittance to the Hegemony of Malis, a conglomerate of civilisations that uses proof of "stability" as the entry requirement.
The Hegemony, however, threatens destruction to any "unstable" civilisations and forcibly incorporates the stable but reluctant....
Blish claims in his introduction to have invented the term "high-energy economy" and his look at how such societies might develop again packs more ideas into 140 or so pages than many novelists' entire output.
Fast-paced fun that makes you think.