When Morse examines the diary, he refers Lewis to the curious incident of the dog that did nothing in the nighttime. He is referring to the Sherlock Holmes story Silver Blaze by Sir Arthur Conan Doyle.
Gina McKee has a tiny role here as the betting shop clerk but she went on to have a big role in the Lewis episode "Old School Ties" playing Lewis' old flame Diane Turnbull.
When Morse says "what about this tramp, Lewis? Where the hell is this tramp?" he pulls from the shelf a copy of "Waiting for Godot" which famously involves two tramps.
Morse takes a copy of Samuel Beckett's 'Waiting for Godot' from a bookshelf when puzzling over the identity of a tramp ('What about this tramp, Lewis?'). In Beckett's play, not only are the characters Vladimir and Estragon usually played as two tramps, but Peter Woodthorpe, who plays the pathologist, Max, played Estragon in the original English production (directed by Peter Hall) in 1955.
The large (Liberty?)-patterned scarf that Ruth (Angela Moray) wears over her coat in this episode is the same as the one worn by Sheila Phillips in "Last seen wearing" (Series 2 episode 2) when she arrives by bicycle to visit Grace Craven unannounced.
Colin Dexter: While Morse talks to the bishop they pass though an archway; to the left Dexter is speaking to a student with a bicycle.