E. Phillips Oppenheim

It's 1913, and war is on the horizon. The disgraced English aristocrat, Everard Dominey, is stumbling through East Africa when he comes across his old classmate and lookalike-the German Baron von Ragastein. Shortly after their chance encounter, Dominey returns to England. But is it really him, or a German secret agent, looking to infiltrate English society ? As Dominey attempts to resume his life, he must reacquaint himself with his insane and murderous wife, the passionate ex-lover that recognizes him, and uncover the mystery of the death that led to his exile.
Oppenheim's classic spy-thriller was enormously popular when it was first published in 1920, selling over a million copies, and leading to three major motion pictures. It is featured on The Guardian's list of "1, 000 Novels Everyone Must Read".