This was the most disappointing pair of Ace Double novels that I have yet read. The main point of interest is that they both shared a common theme of sci-fi "supermen," albeit in different circumstances.
The first one I read was Eric Frank Russell's Three to Conquer. In it, a precision instruments maker in the near future who happens to be telepathic stumbles across an alien plot to take over humanity. The idea of an alien virus being able to take over terrestrial life forms is pretty sinister, as it is virtually undetectable by humans, but in the end it serves mainly to give Russell's protagonist the ability to serve as the hero by telling cops and FBI agents how to do their job. It's suspenseful, but the ending is disappointingly anticlimactic.
By contrast, Robert Moore Williams's Doomsday Eve is anything but gripping. His story begins with soldiers fighting in a futuristic third World War encountering frequent interventions by "new people" who demonstrate remarkable superpowers. An intelligence officer assigned to investigate them finds out about their mission to save humanity and the impending effort by the "Asiatics" to destroy the continent. Williams telegraphs his ending practically from the book's early pages, leaving much of the book feeling like a wheel-spinning exercise as a result.