"The Prisoner of Zenda" by Anthony Hope

Well, this was a jolly romp of a tale!  Kind of a grown-up version of The Prince and the Pauper, in a way, but with the switch of commoner and royalty being deliberate and done for an actual reason.
Rudolf Rassendyll, a wealthy young man idly wasting his life at gentlemanly nonsense, goes to visit distant relatives in the middle-European (and fictional) country of Ruritania.  There, he meets another Rudolf, his distant cousin who is about to be crowned king.  After a night of drunken revelry, the...