The Lord of the Rings

 was either a book or a series of books that I.M. Foreman had been reading shortly 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)

Astrolabus forced the Sixth Doctor to quote the book in The Cabinet of Astrolabus. (DWM: Once Upon a Time Lord)

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 had a relationship with. (EDA: The Taint)

The Tenth Doctor and Rose Tyler also were familiar with the characters from Rings. Rose referred to Chip as "Gollum" because of similarity in appearance and stature to that character. (DW: New Earth) Later, the eleventh incarnation of the Doctor jokingly called himself "the outer space Gandalf". (DW: Meanwhile in the TARDIS)