The covers for this Ace Double book are on the spectacular side. One, by Ed Valigursky, depicts a spaceman fleeing on foot. Pursuing him is what looks like a giant razor-edged roulette wheel, rolling on its edge. It is cutting a groove in the soil and looks as if it is about to slice the spaceman in half. The cover to the flip side of the book, by Ed Emshwiller, shows a mass of machinery. Superimposed over the machinery is the face of a green-skinned alien. And superimposed over _that_ is the face of a human. A lovely young woman in an evening gown stands in the background.
The novels are William F. Temple's _Battle on Venus_ and Robert Silverberg's _The Silent Invaders_ (both 1963). They represent, I fear, two good authors turning out a couple of potboilers. Neither novel is a complete stinker. But both are readily forgettable. I believe that the Silverberg is marginally the better of the two.
_Battle on Venus_ was probably written just before the findings of the Mariner space probes. It involves the first spaceship (manned) to land on Venus. There have been no previous rocket probes. Venus proves to have a breathable atmosphere. There are also plains, mountains, caves, and glaciers*. It is populated with humanoid aliens who can conveniently communicate with the Earthmen by telepathy. Some of the aliens are beautiful girls who are ripe for romantic involvement with some of the Earthmen. The spaceship lands on a plain in the middle of a perpetual war fought with the pit-and-the-pendulum wheels, shells, tanks, planes, and gigantic war machines. There are a lot of captures and escapes before all is happily resolved.
The Silverberg involves an alien race who have surgically altered themselves in such a way as to pass for human. They are infiltrating Earth with plans to take it over. But gradually the hero, one of the alien spies, finds that he is becoming more and more human. Eventually he goes native, and the invasion fizzles out. Silverberg is a good enough writer to almost convince you that an alien can become a human when he looks and acts like a human. But he does not completely persuade.
This pair of novels must be considered one of Ace's below par entries.
* I know of no other science fiction novel or story that assigns glaciers to Venus. Seas, yes. Swamps, yes. Jungles, yes. Deserts, yes. But glaciers, no.