The Dark Frontier by Eric Ambler

"The Dark Frontier "launched Eric Ambler's five-decade career as one of the most influential thriller writers of our time.
England, 1935. Physicist Henry Barstow is on holiday when he meets the mysterious Simon Groom, a representative for an armaments manufacturer. Groom invites the professor to Ixania, a small nation-state in Eastern Europe whose growing weapons program threatens to destabilize the region. Only after suffering a blow to the head--which muddles his brain into believing he is Conway Carruthers, international spy--does the mild-mannered physicist agree to visit Ixania. But he quickly recognizes that Groom has a more sinister agenda, and Carruthers is the only man who can stop him.

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