The more Ace Doubles I read, the more I come to appreciate how varied the experience of reading them can be. For all of their similarity of their size, their plot-driven approach, and their cover art (which typically consists of square-jawed white dudes inflicting violence on aliens or some other evildoers, often with a woman somewhere in the scene recoiling in terror), the quality and nature of the books can vary widely.
This pair provided the best reflection yet of these differences. Ray Cummings's The Man Who Mastered Time was unusual in that it was not an original work but a reprint of a 1920s story which reads like a riff on H.G. Wells's famous novelette The Time Machine. In it, a father-and-son duo of scientists stumble across a process that allows them to peer into the indeterminate future. Witnessing a beautiful girl imperiled by a thuggish brute, the two turn a hoverable aeroplane into a time machine, which the hormonally-driven son uses to travel thousands of years into the future to rescue the maiden. He soon finds himself in the midst of a political struggle between the people of an ice-age north and the remaining civilization, which has retreated to the Caribbean and reflects a class divide that ol' Herbert George would have found familiar (seriously, it wouldn't surprise me in the least to find that he sued for copyright infringement). The young man soon summons his father for aid, and with the help of a friend, aid the civilized underdogs against the barbarian hordes. There are some aspects of the novel – such as the employment of "girls" in combat – that but for the most part it's a prime piece of pulp science fiction, and while it had it's share of problematic elements (the scientist's friend zeroing in on the beautiful girl's teenage sister seemed a little predatory even for the time) I enjoyed it for the action adventure it was.
The other novel was Joseph Kelleam's Overlords from Space. Here there was a real contrast with Cummings's novel; whereas Cummings has heroic adventurers as his protagonist, Kelleam's novel centers around humans enslaved by the Zarles, an alien species who conquered the Earth two centuries before. Though their domination of the Earth seems absolute, the ostensibly immortal Zarles are slowly dying from terrestrial disease. Worse they cannot reproduce, and the remaining Zarles are contemplating destroying the Earth and moving on elsewhere. It's a different premise from the ones I expect from the time, though the plot itself moves to familiar beats involving freedom, the discovery of resources and allies that can even the odds, and a climactic battle in which the outcome isn't really in doubt. In this respect it's as much a product of its time as Cummings's older novel (which ends, I kid you not, with a Jazz Age party), though one that proved entertaining enough to see through to its end.