(This review may be considered to contain spoilers.)
"Negroes always have large feet."
This novel is outstanding for its prejudiced (period) attitudes.
"Will the test show whether it's a white person's skin or a nigger's?"
On the same theme:
"In the meantime, I wish you'd have his finger nails carefully cleaned. I want——"
But the request had instantly overwhelmed Greenleaf.
"What!" he yelled. "Clean his finger nails!"
"Yes," Bristow continued smoothly, disregarding the other's evident distaste and surprise. "If I were down there, I'd do it myself. In fact, it would be better for you to do it. Don't leave it to some careless subordinate."
The chief laughed his sarcasm.
"You know," this still with laughter, "we Southerners are none too strong on acting as manicures to these coloured folks."
A happy neologism:
"An' I went on back to de bellboys' bench and stahted in niggerin' it once mo'e."
"Yas, boss; you know, dat means quick sleepin'."
Whether this means niggardly naps of short duration or falling asleep quickly, I do not know. I suspect the latter, but James Hay Jr. scholars of the 24th century may interpret it as the former and change the spelling accordingly.
Another aspect is the negro fear of the supernatural:
She sat up again and began to think about Mrs. Withers, how well the slain woman had treated her, how kindly. From that, her thoughts went to ghosts. She fell to trembling and moaning in an audible key. It was not long before a warden, awakened by her cries of terror, had to visit her and threaten bodily punishment if she did not keep quiet.
Traditionally, this fear is attributed to the slave owners' deliberate cultivation of superstition as a medium of control, usually by stories, occasionally by masquerade. In the context of the novel, it's simply a familiar comic subject.
To be completely fair, an incredibly brutal and distasteful scene is, in fact, a clue, and does not have the sympathy of the author (although it is difficult for the reader to identify it as such given the general trend).
On other matters, the novel has an audacious twist, possibly novel enough for its time, although certainly done better by later authors. This is not adequately clued by fair-play standards. There are no physical or psychological clues with a (necessary) logical interpretation leading to the culprit. There are, however, numerous indications of character which keep suggesting the possibility of a surprise in a certain direction, some of which are very clever (like the confounding racially charged situation described above). The major problem is that the impersonal third person narrator makes several statements which are later found to be untrue, a technical infelicity Agatha Christie would have avoided.