The Prisoner of Zenda & Rupert of Hentzau

The king is poisoned and replaced by an imposter You know the story because you've seen it everywhere from the movie Dave to the Robert A. Heinlein novel Double Star, but that plot started right here with the Anthony Hope classic, The Prisoner of Zenda

The sequel to The Prisoner of Zenda and picks up three years after the end of the first book. Queen Flavia writes to her love, Rudolf Rassendyll, and the letter is to be hand delivered by Fritz von Tarlenheim, but Rupert of Hentzau steals it because it's his best chance of getting back in with the king. Rassendyll goes back to Ruritania, and is once again put in the position of impersonating the king. Violence and adventure ensue as Rupert and Rassendyll face off.