What is the theme in The Demon Lover by Elizabeth Bowen?

The theme of appearance and reality is central to "The Demon Lover." The strange appearance of the letter puzzles Mrs. Drover. There are many questions about its authenticity as well as how it was accessed into the house. Her house is obviously deserted and untouched, which makes the appearance of the letter even more dubious. Is the letter true? Is it part of her reality?