Mysteries featuring Letters (3 books)

My favorite mysteries in which a letter features as a key clue or plot point

flag

1	The Purloined Letter (C. Auguste Dupin, #3) by Edgar Allan Poe 3.62 avg rating — 14,079 ratings score: 200, and 2 people voted	Error rating book. Refresh and try again. Rate this book Clear rating 1 of 5 stars 2 of 5 stars 3 of 5 stars 4 of 5 stars 5 of 5 stars
2	A Letter of Mary (Mary Russell and Sherlock Holmes, #3) by Laurie R. King (Goodreads Author) really liked it 4.00 avg rating — 21,555 ratings score: 99, and 1 person voted	Error rating book. Refresh and try again. Rate this book Clear rating 1 of 5 stars 2 of 5 stars 3 of 5 stars 4 of 5 stars 5 of 5 stars
3	Lord Edgware Dies (Hercule Poirot, #9) by Agatha Christie 3.95 avg rating — 61,825 ratings score: 98, and 1 person voted	Error rating book. Refresh and try again. Rate this book Clear rating 1 of 5 stars 2 of 5 stars 3 of 5 stars 4 of 5 stars 5 of 5 stars