Pope Celestine V, aka Peter Morrone, quit in 1294. He was the only pope, prior to Benedict XVI, to outright quit. (Gregory XII resigned in 1415 to end a schism, amid a debate about popes and anti-popes.
Well, Celestine V was immortalized in an excellent book by Jon Sweeney a couple years ago, The Pope Who Quit: A True Medieval Tale of Mystery, Death, and Salvation:
At the close of the tumultuous Middle Ages, there lived a man who seemed destined from birth to save the world. His name was Peter Morrone, a hermit, a founder of a religious order, and, depending on whom you talk to, a reformer, an instigator, a prophet, a coward, a saint, and possibly the victim of murder. A stroke of fate would, practically overnight, transform this humble servant of God into the most powerful man in the Catholic Church. Half a year later, he would be the only pope in history to abdicate the chair of St. Peter, an act that nearly brought the papacy to its knees. What led him to make that decision and what happened afterward would be shrouded in mystery for centuries. The Pope Who Quit pulls back the veil of secrecy on this dramatic time in history and showcases a story that involves deadly dealings, apocalyptic maneuverings, and papal intrigue.
If you’re a church history nerd like me, pick it up.