Two Questions (spoilers)
1) How did Rupert get the rope at the end of the movie?
2) What do the flashing neon lights on the building next to the apartment symbolize?
1) I think Rupert explained to David's father that the rope wasn't holding the books together, and he offered to take the rope back to the owners. David's father wouldn't have seen anything odd about that.
2) Nothing IMHO.
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Jim Hutton (1934-79) & Ellery Queen =