Earth's Last Citadel by C. L. Moore & Henry Kuttner (1964)
The husband/wife team of Kuttner and Moore (or in this case, the wife/husband team of Moore & Kuttner) were seamless in their many collaborations. Bibliographers are unable to come up with a definite list of who wrote what under their many pseudonyms. Both were talented writers in their own right and both churned out number of classic stories for the pulps, although Kuttner in particular was not adverse to producing sub-par tales if the market called for it.
My Forgotten Book this week began as a four-part serial in Argosy in 1943 and was reprinted in the July 1950 issue of Fantastic Novels, Donald A. Wollheim published it as a paperback in 1964 from Ace, which reprinted it in 1977 and in 1983.
Army Intelligence officer Alan Drake is tasked with smuggling scientist Colin Douglas out of Tunisia while being pursued by two determined Third Reich agents -- red-headed Karen Martin and ruthless Mike Smith, neither one blessed with a Nazified name. The German agents catch up with them on the Tunisian coast just as Alan and Colin stumble across a large meteor that had crashed the night before. Of course it's not a meteor; it's a space ship. Suddenly all four go into a robot-like trance and as a door appears on the side of the ship, they enter it. The ship then buries itself in the sand.
Fast forward a billion years or so. Alan wakes up with vague memories of a strange presence that would occasionally be watching him. The others soon awaken and the whole Nazi vs. Allies thing starts up again...until they exit the ship and find themselves on a dying world pursued by the amorphous presence that had been on the ship with them. This thing, which they dub the Light-Wearer, is a sort of psychic vampire, leeding off the energy of living things. It is a member of an alien race that had invaded Earth and destroyed the human race, keeping only a few as "pets." They performed all sorts of genetic and biological experiment on these remaining humans, creating a number of weird races. Then, over millions of years, the alien race died out, leaving a number of of eerily designed citadels. By the time the four Twentieth Century humans arrive, only one citadel remains.
Also remaining are two human races: the ethereal Carcasillians who occupy the citadel and are ruled by Flande, a megalomaniacal psychic, and the savage Teresi, a small band of maybe a hundred who appear to be humanity's last hope. The Carcasillians are immortal in a way, taking their immortality in chunks by renewing themselves in a fountain that is powered by a mysterious source. As a final battle looms between the Carcasillians and the Teresi, and as the Light-Wearer waits in the wings to destroy what remains of humanity, Alan must find a way to save the human race as well as the lovely Evaya, the Carcasillian he loves.
Pure pulp that moves along at a thundering speed, with plot holes aplenty and an A. Merritt sensability. They just don't do this type of thing this well anymore. I loved it.