John novel that was based on a medieval Mexican folktale?

You're likely thinking of "The Ballad of the Sad Cafe" by Carson McCullers. It's not technically a *novel* but a novella, and while it's not directly *based* on a Mexican folktale, it does draw heavily from the structure and themes of a Mexican folk ballad known as the "Corrido."

The *corrido* is a narrative ballad often focusing on love, loss, revenge, or outlaw figures. McCullers uses the *corrido's* structure and themes to tell the story of a lonely woman named Miss Amelia, her relationship with the dwarf, and the arrival of a man named Marvin Macy that disrupts their lives.

While not directly based on a specific Mexican folktale, "The Ballad of the Sad Cafe" certainly reflects the spirit and storytelling style of the *corrido* in its structure, its focus on a specific community, and its use of themes and characters that resonate with folk traditions.