Complex sentence: The book itself was allegedly based on an anecdote told to the painter Walter Sickert by his landlady when renting a room; she said that the previous tenant had been Jack the Ripper.
Split into simpler sentences: The book itself was allegedly based on an anecdote told to the painter Walter Sickert by his landlady when renting a room. The landlady told Sickert that the previous tenant had been Jack the Ripper.