The Great Impersonation (Barnes & Noble Digital Library)

This 1920 novel, a marvel of taut plotting, is regarded as among the finest spy stories of all time.  Just before World War I, a dissolute English nobleman and a German secret agent, who happen to resemble each other exactly, meet by chance in South Africa.  The German decides to take the Englishman's place and return to England in order to gain political power in conflict-filled Europe.