Frankie's Letter by Dolores Gordon-Smith

On a rainy day, Terrace Cavanaugh, an Irishman, American spy walked in a staggard fashion towards his comrade house Anthony Brooke in Germany.Before dying he gave a chunk of confidential information to Brooke speaking vaguely like "I love her and something about A gentlemen, Read frankie's letter . . ."

Sir Charles, the mastermind behind the working of Intelligent Service works with Brooke to solve a war plans of Germany impending leading to American World War 1 using Terrace information. All the clues imply to only a single location and from here the thrilling adventure of Brooke's begins.