Peter Rabbit

Peter Rabbit was the central character in a human childrens' story that K9 Mark II once read to the Fourth Doctor. Romana II, overhearing the reading, interjected that there was no need for the Doctor to be concerned about Peter's safety, as she already knew that Peter would escape the clutches of Mr McGregor. (TV: The Creature from the Pit)

Salyavin kept a first-edition copy of Peter Rabbit in a bookcase in his study. (PROSE: Cambridge Previsited)

Behind the scenes
Given the ending Romana recounts, and indeed the given quote, K9 would appear to have been reading from.