"The Key to Irunium" is a novel by British writer Kenneth Bulmer, published in 1967 as half of an Ace Double. Kenneth Bulmer was a prolific genre writer in both science-fiction and other types of adventure stories, and this book was part of an eight book series about dimensional travel, called "The Keys to the Dimensions".
This book has a more ambitious story structure than the average Ace Double. It uses the story device of "average person magically transported into a world of adventure", which involves having a bit of set-up of "average life" before starting the science-fiction or fantasy elements. Robert Infamy Prestin is an aviation journalist who has always noticed things around him seem to get lost. It is a minor problem, until one day, on a jet plane flight into Rome, a young woman he is talking to disappears. Soon, two competing groups are trying to get control of Prestin, leading to some James Bond style chapters where helicopters and sports cars fight gun battles on the Italian coast, before Prestin dimension hops into a world where high technology coincides with sword and sorcery. (Mixing high and low technology being a habit of Ace Doubles). After several shifts of scenery, the book is concluded, at least for the time being, until the series resumes.
Kenneth Bulmer was an experienced writer, which is perhaps why he was able to pull off a more complicated story structure. To use the venerable wardrobe method of fantasy, where a normal person finds a portal into another dimension, the author first has to spend some time establishing the "normal" circumstances, and then, when the transition happens, has to demonstrate the psychological shock of being hunted down by "Trugs" for a normal person. Followed by two other scene transitions in the book's 138 pages. This is all handled competently, if not spectacularly.
Although I enjoyed this story, it seemed to be a bit more cynical, violent and sexist than other Ace Doubles. Although I am working with a short sample, I have found that the zeitgeist of earlier, late 50s and early 60s "rocketship and raygun" science-fiction is, even when sexist and dated, somewhat charming, while the late 60s and early 70s come across as just a little bit greasy. This book has many more depictions of violence and torture than I found in earlier Ace Doubles, such as "The Pirates of Zan" or "The Ship from Outside", and it also in a few passages seemed to glorify the idea of a self-sufficient "uberman". It basically read as the worst stereotypes of Heinlein, without the redeeming quality of having original ideas. It was nothing that distracted from reading it, but its attitude seemed more dated, and less charmingly so, than the more "gee whiz" space operas that proceeded it.