The tone of the novel is ironic and sometimes a bit smug, usually in a playful way. One early example is in Chapter 1, when Dr. Sheppard describes his sister, Caroline, as a gossip:
The motto of the mongoose family, so Mr. Kipling tells us, is: “Go and find out.” If Caroline ever adopts a crest, I should certainly suggest a mongoose rampant. One might omit the first part of the motto. Caroline can do any amount of finding out by sitting placidly at home. I don’t know how she manages it, but there it is. I suspect that the servants and the tradesmen constitute her Intelligence Corps. When she goes out, it is not to gather in information, but to spread it. At that, too, she is amazingly expert.
Sheppard is alluding to a story by Rudyard Kipling called "Rikki-Tikki-Tavi," which is about an adventurous mongoose. There is no reason he could not simply state that Caroline likes to gossip. Instead, he uses this allusion to signal that he is well-read and then, in a roundabout way, to draw a distinction between his refined mind and his sister's busybody nature. He uses verbal irony, pretending to elevate her to the status of an "expert" who runs her own "Intelligence Corps" of gossipers. Ultimately, beneath the veneer of veneration, he is comparing her to a mongoose and suggesting that her interest in the neighbors' business is laughable.
This passage is both funny and mean. Sheppard clearly thinks he is better and smarter than Caroline. There is a bit of a distinction to be made between Sheppard's tone and the novel's tone; after all, Sheppard is not only the narrator, but also a character in the novel. But Christie is likewise ironic and a bit above her readers and characters—even Sheppard. She may not wholeheartedly agree with Sheppard's assessment of his sister, but she seems at least in part to agree that gossiping is a guilty pleasure, much like mystery novels. What's more, Sheppard's words allow Christie to make the opposite point about him. Sheppard thinks Caroline's gossip is frivolous, but it helps Poirot identify him as Roger Ackroyd's killer. He is not as smart as he thinks. Christie refers to the genre of detective fiction enough to insert herself quietly as the clever mystery writer behind the plot. The only character who comes close to being as smart as her is Poirot himself.
