The Lord of the Rings

 was either a book or a series of books that I.M. Foreman had been reading soon before meeting the Eighth Doctor. She claimed that Rings was very long. When the Doctor mentioned travelling with someone named "Sam", Foreman was reminded of a character from Rings. The Doctor, for his part, was reminded that Fitz had been talking about Rings. (EDA: Interference - Book One)

While working at a garden centre, Fitz had a conversation with a woman and convinced her that the character of Frodo had been named after a woman the author, J. R. R. Tolkien, had a relationship with. (EDA: The Taint)