The Prisoner of Zenda by Anthony Hope
Anthony Hope's swashbuckling romance transports his English gentleman hero, Rudolf Rassendyll, from a comfortable life in London to fast-moving adventures in Ruritania, a mythical land steeped in political intrigue. Rassendyll bears a striking resemblance to Rudolf Elphberg who is about to be crowned King of Ruritania. When the rival to throne, Black Michael of Strelsau, attempts to seize power by imprisoning Elphberg in the Castle of Zenda, Rassendyll is obliged to impersonate the King to uphold the rightful sovereignty and ensure political stability. Rassendyll endures a trial of strength in his encounters with the notorious Rupert of Hentzau, and a test of a different sort as he grows to love the Princess Flavia.