A Spy in the Shadows [Spy Noir Series #1] by Randy Grigsby [AZW3+]

November 1943. Tehran, Iran. The three allied leaders, Winston Churchill, Franklin Roosevelt, and Joseph Stalin meet to discuss the invasion of Europe and final defeat of Nazi Germany. When a British Intelligence officer is murdered just days before the start of the conference, Booth Salinger, an American OSS officer, is sent to Tehran to investigate. Against this backdrop A Spy in the Shadows unfolds exposing a tale of dedicated opponents, betrayal of allies, and a German spy codenamed Traveler who possesses secrets that could delay the allied invasion and change the outcome of the war.