Keith Laumer is one of the unfairly forgotten luminaries of the Silver Age of Science Fiction. THE OTHER SIDE OF TIME is a solid, well-written and fairly straightforward work of sideways-in-time style adventure.
The Silver Age is the generation of science fiction after the John W Campbell Jr stable of authors gave way to a new breed of authors: Jack Vance, Poul Anderson, Jerry Pournelle, Larry Niven, Roger Zelazny, James Gunn, Alfred Bester, Philip K Dick, Gordon R. Dickson and Frank Herbert ranged beyond the boundary of magazine sales, and won fame in paperback and even hardback markets.
The science fiction audience in the Silver Age had over two decades of familiarity with the odd assumptions of this unique form, and so could be safely assumed to be familiar with the tropes of science fiction. A very rough consensus of future history had emerged, which assumed spaceflight and interplanetary colonization were ahead of us, and the years to some were peopled with young interstellar federations and old galactic empires powered by atomics and crossed by hyperdrive, infested with spies or special agents armed with ESP.
An earlier generation of readers would have asked for some clear explanation of these marvels to aid their suspension of disbelief; the postwar generation who had seen the V2 rockets over London needed not to hear them again. This gave the writers of that age the ability within a shorter space to cut some nuts-and-bolts and add more other matter: and Keith Laumer added fast, lean, mean and masculine action like a science fiction version of Dashielle Hammet or Raymond Chandler.
Laumer is best remembered for his wry James Bondian satires of Retief, Diplomat Extraordinaire, who continually undoes the boneheaded folly or petty evil of his superiors in the diplomatic service, ever eager for preemptive surrender, and to aid and abet the enemies of Terra. But I myself prefer his more serious work, which range from somewhat lighthearted action story to the somewhat grim action story.