The Winning Clue, by James Hay, Jr. (1919)

(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."

"Niggering it?"

"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.

Five Little Pigs: a small unimportant matter

Shortly before the dénouement, Poirot asks each of his "pigs" a question. For Angela Warren, the "question he did want to ask her could wait", so he improvises a different one:

 “At the time of the tragedy, you had lately read, had you not, Somerset Maugham’s The Moon and Sixpence?”

Angela stared at him. Then she said:

“I believe—why, yes, that is quite true.” She looked at him with frank curiosity. “How did you know?”

“I want to show you, mademoiselle, that even in a small unimportant matter, I am something of a magician. There are things I know without having to be told.”

The connection to The Moon and Sixpence is not obvious, and in fact is never explained in the novel. This blog post attempts to uncover Poirot's reasoning.

 Briefly, The Moon and Sixpence is a novel (1919) by Somerset Maugham based on the life of Gaugin. The main character, Strickland, leaves his family to become an artist, eventually dying of leprosy.

  Angela's literary tastes are mentioned in her narrative:

  Besides, just at that time, I’d suddenly begun to discover the intoxication of words. Things that I read, scraps of poetry—of Shakespeare—would echo in my head. I remember now walking along the kitchen garden path repeating to myself in a kind of ecstatic delirium “under the glassy green translucent wave”…It was just so lovely I had to say it over and over again.

Even though her recollection of Comus is faulty*, it is clear that the literature she reads is bubbling over into her everyday mental life.

Miss Williams' narrative brought out that:

Angela ... was also a passionate reader and showed excellent taste in what she liked and disliked.

 We can assume that in the 1920s (when the events surrounding Amyas Crale's death take place) that the popular Maugham's The Moon and Sixpence would be something a literary young woman of excellent taste might have read. Had Angela done so, she could not fail to have identified Crale with Strickland - in his artistry, his selfishness, his sexual conquests. The clinching connecting clue is in Philip Blake's narrative:

Amyas and Angela had a first-class row, I know, and the rest of us rather welcomed it. It avoided other difficulties. Angela rushed off to bed with a final vituperative outburst. She said A, she’d pay him out. B, she wished he were dead. C, she hoped he’d die of leprosy, it would serve him right. D, she wished a sausage would stick to his nose, like in the fairy story, and never come off. When she’d gone we all laughed, we couldn’t help it, it was such a funny mixture.

Point C, together with Angela's anticipated reading and her reading's tendency to break into her thoughts, seems to have formed the basis for Poirot's guess (although he artfully, and quite rightly, claims to know).

* The phrase has 'cool' for 'green' in Comus but is reproduced as Chrisitie has it in Wet Magic by Edith Nesbit. We may note the children in Wet Magic read The Water Babies at the beginning of the novel and, of course, Amyas is named so from a character in Kingsley's Westward Ho! Perhaps Poirot could deduce what Christie read to her daughter?