In the last seconds of Dead Letters we see, on a book rack in the small lending library, a copy of Chromosome Wars by Jezebel Tripp, the author we met in Sins of Commission (2004) from season 7.
Two actors from this episode starred in the classic romantic comedy Four Weddings and a Funeral (1994), Simon Callow and Sophie Thompson.
Richard Cant plays Alistair Gooding in this episode. He previously played the role of Dennis Rainbird in episode 1.1, The Killings at Badger's Drift (1997).
Elizabeth Spriggs plays Ursula Gooding in this episode. She previously played the role of Iris Rainbird in episode 1.1, The Killings at Badger's Drift (1997).
The second and final appearance for Barnaby's grey Jaguar X-Type (BL 54 BWC) before having a newer model in s9e3.