A groundbreaking fantasy when originally published in 1953, this Anderson work broke ground that has been tread and retread by many others, and thus will seem derivative to contemporary readers.
The hook: a modern-day man is catapulted into a world where magic is real and finds he is playing a crucial role in a battle between the forces of good and evil.
Yeah, we've only read that one about a billion times.... Let's see. Narnia. Joel Rosenberg. One of the Xanth books. The Spellsinger series. Etc. The difference is that this guy is an engineer fighting in World War II (this one goes way back, people). Anderson did it first.
That doesn't mean it's very good. It's reminiscent of Heinlein's Glory Road in that the protagonist has some fun things happen to him, wanders around an interesting landscape, and then saves the day, with much lack of direction and coherence. The wish-fulfillment is laid on pretty strong, as Holger Carlson finds himself stronger, taller, and more attractive to women of all races than he was in our universe. The female lead, Alianora, is underdeveloped and completely uninteresting. Worst of all, the big payoff - the mysterious identity of Holger's other self - is a complete wash-out.
Interesting for context of F/SF history; otherwise not recommended.