Agatha Christie over reworked the plots of older short stories into full novels. "Endless Night" (1967) reused the plot of the short story "The Case of the Caretaker" (1942). The original short story featured Jane Marple, the 1967 novel does not.
The original 1967 novel has Michael Rogers as the narrator of the story and does not feature Miss Jane Marple at all.
The fortune the Gipsy tells, which includes the episode title "Endless Night," is actually just a section of the William Blake poem "Auguries of Innocence."
The original novel was more of a thriller/tragedy rather than the murder mystery this adaptation turns it into.
The character of architect Robbie Hayman was based on the source novel's character Rudolf Santonix.