At the end, when Charles Arundel states that he's leaving water speed records to "Campbell and his cronies," he's referring to Malcolm Campbell (1885-1948), who set several land and water speed records. In his craft "Blue Bird K4" his highest water speed record was over 141 MPH (227 km/h), a mark reached in 1939.
The book Wilhelmina is reading in bed is an omnibus edition of novels by HG Wells called "Stories of men and women in love." it includes four novels: Love and Mr. Lewisham, The secret places of the Heart, The Passionate Friend, and The Wife of Sir Isaac Harman. Most 21st-century readers are not aware that Wells wrote books besides science fiction novels.
The portrait painting of General John Laverton Arundell also features in The Mystery of the Spanish Chest (1991) in two scenes within the Military Club.
In the scene in the club, the song in the background is By the Fireside, sung by Al Bowlly with Ray Noble's Orchestra. Bowlly was a very popular ballad singer in the 1930s, the period in which most of the Poirot mysteries are set. Bowlly was killed in April 1941 by a Luftwaffe parachute mine as he returned home from a gig.
Relaxing outside the hotel, Poirot's choice of reading is a book of Wordsworth's poetry; Hastings is reading The Times.