Here are the most valuable books I’ve found offering insight into “The King o’ the Cats,” one of my favorite English folktales, along with its many variants in other cultures. Most links are for more info at Amazon.com, an affiliate.
King o’ the Cats, by Aaron Shepard, Atheneum, New York, 2004. My own retelling of the story, much expanded from the original!
More English Fairy Tales, collected by Joseph Jacobs, Nutt, London, and Putnam, New York, 1894; reprinted by Schocken, New York, 1968. Jacobs provides what is now the standard and best-known version of the tale, plus a background note.
Irish Folktales, edited by Henry Glassie, Pantheon, New York, 1985. Includes several tales of the cat king.
Hamlet’s Mill: An Essay on Myth and the Frame of Time, by Giorgio de Santillana and Hertha von Dechend, Gambit, Boston, 1969. Chapter 21 discusses versions of the tale from many cultures, starting with Plutarch’s.
Funk & Wagnalls Standard Dictionary of Folklore, Mythology, and Legend, edited by Maria Leach, Harper & Row, 1972. Includes a brief entry on “king of the cats,” focusing on the Irish legend of Irusan. The entry on “Pan” notes the adoption of Plutarch’s story into Christian legends of the Crucifixion.
Brewer’s Dictionary of Phrase & Fable, Sixteenth Edition, HarperCollins, New York, 1999. The entry on “Pan” notes the adoption of Plutarch’s story into Christian legends of the Crucifixion.
Plutarch’s Moralia, Volume 5, translated by Frank Cole Babbitt, Harvard University, Cambridge, Massachusetts, and Heinemann, London, 1936, pp. 399–403. The oldest known version of the tale, regarding the death of Pan.
Plutarch’s Morals: Theosophical Essays, translated by C. W. King, George Bell, London, 1889, pp. 92–93. An older, more archaic translation.