•Why it earns a slot
Why it earns a slot: it is the book that invented the political-doppelganger plot wholesale, the Ruritanian romance genre still gets named after this fictional country, and its central trick, a commoner crowned by accident who then has to keep proving himself worthy of a throne he cannot legally hold, reads as a clean allegory for anyone forced to perform authority they were never granted.
An idle English gentleman who happens to share a king's face gets drugged royalty, a kidnapped monarch, and a throne he was never supposed to sit on.
This distillation is written from the freely available original, which is always the better read when you have the time: Project Gutenberg.