THE WARRIORS OF DAY
The first mass-market paperback of a very early novel by James Blish: The Warriors of Day. This originally appeared as "Sword of Xota" in Two Complete Science-Adventure Books June 1951, and there was a GALAXY-Novel edition in 1953.
[It has been so long since I read my copy that I'll just quote the back-cover copy and first-page blurb, to give some idea of the set-up...]
[The first-page blurb]: A LITTLE KNOWLEDGE... One thing was clear: Earth and Xota had virtually nothing in common - except Tipton Bond. Soberly Tipton attempted to extract from his sojourn in the temple the few positive things that he had learned. There was not much to go on. There was the voice, the presence, and its reference to himself. There was the name of this world, Xota. There was an entity named "Mahrt" and a "Sword," which he had surrendered or was about to surrender. Finally there was a group called the Warriors of Day, about which Tipton had overheard nothing but a vague suggestion that they were approaching, and that they were enemies... Wait a moment - the Warriors wer enemies of Xota. If "Xota" was the whole world, the Warriors of Day must be approaching from the outside - and their ships must be - Spaceships.