City of a Thousand Suns by Samuel R. Delany

In a city halfway across the galaxy an alien intelligence was addressing the strangest delegation since time began: "We have called you here to help us. Our universe has been invaded by the Lord of the Flames. He has been gathering information on many worlds for a full-scale attack. He is now preparing his final effort to make the planet Earth his initial beachhead..."
On Earth, the Empire of Toromon, the highest hope of a radioactively blasted world, was unaware of the danger. But three persons within the empire suspected, the three who together would bear the brunt of a defense against a monster antagonist that had never given or asked quarter.

Condition: Acceptable. Rubbing and discoloration.