The Hunting of the Snark

 was a work by Lewis Carroll that Peri once used to help Erimem learn how to read the English language. She read it aloud as they travelled on top of the train from Siliguri to Darjeeling en route to a cricket match. (BFA: The Roof of the World)

Behind the scenes
The Seventh Doctor short story, The Hunting of the Slook, takes its name from this work.