The Dead Letter (Musaicum Vintage Mysteries)

Richard is set to join a new law firm owned by his friend's (Eleanor's) father. Inevitably he is dragged into the murder mystery of Eleanor's fiancé Henry. With detective Burton in tow, Richard must find the clue that is hidden in plain sight as a "dead letter" in the post office!
Excerpt:
"I paused suddenly in my work. Over a year's experience in the Dead Letter office had given a mechanical rapidity to my movements in opening, noting and classifying the contents of the bundles before me; and, so far from there being any thing exciting to the curiosity, or interesting to the mind, in the employment, it was of the most monotonous character.Young ladies whose love letters have gone astray, evil men whose plans have been confided in writing to their confederates, may feel but little apprehension of the prying eyes of the Department; nothing attracts it but objects of material value—sentiment is below par; it gives attention only to such tangible interests as are represented by bank-bills, gold-pieces, checks, jewelry, miniatures, et cetera..."