Just Once by Karen Kingsbury

The #1 New York Times bestselling author "known for her deeply heartfelt novels" (Woman's World) and the Baxter Family novels writes a sweeping World War II love story about a young woman torn between two brothers.
In 1941, beautiful Irvel Holland is too focused on her secret to take much notice of the war raging overseas. She's dating Sam but in love with his younger brother, Hank—her longtime best friend—and Irvel has no idea how to break the news. Then the unthinkable happens—Pearl Harbor is attacked. With their lives turned upside down overnight, Sam is drafted and convinces Hank to remain in Indiana, where he and Irvel take up the battle on the home front.

While Sam fights in Europe, an undeniable chemistry builds between Irvel and Hank but neither would dare cross that line. Then, two military leaders pay Irvel a visit at the classroom where she teaches. The men have plans for her, a proposition to join a new spy network. One catch: She can tell no one.

With Irvel caught between two brothers thousands of miles apart, can love find a way, even from the ashes of the greatest heartbreak?