I'll Be Seeing You - Eileen Charbonneau

Luke Kayenta and his childhood friend Nantai Riggs are young shepherds of the Navajo reservation in Arizona. They volunteer for an experiment: to come up with an uncrackable code based on their language to be used by the US as it enters World War II. They fly into New York to join the spy agency the Office of Strategic Services (OSS). While on the airfield, Luke catches sight of a young woman. He is first enchanted, then heartsick when he finds that Kitty Charante is the devoted wife of his Canadian RAF pilot and instructor in espionage. Their paths will cross again. 

In the mountains between Spain and Nazi-occupied France, Luke and Nantai practice the code between radio stations while helping on Allied missions. 

But off the coast, German SS agent Helmut Adler is celebrating his latest U-Boat conquest when he hears something crackling over the radio lines. When a young soldier of World War I, an American Indian language helped bring down his regiment in the Battle of Meuse-Argonne. Adler swears it will not happen again in this war. He sets his sights on the code talkers. 

The hunt has begun.