...is Sir Thomas Malory's La Morte d'Arthur.
It is the classic tale of the Athurian Legends and perhaps the greatest stories told of King Arthur and the Knights of the Round Table.
This collection of stories covers nearly every Knight and is the epitome of the King Arthur legend.
It is what true classical Romance is supposed to be and is a great read by any measure: Medieval or Mordern.
If you've not had the chance to read it, it is a must for anyone trying to understand Romance.