As Billy is settling into the bed and breakfast and sipping on some tea that the landlady prepared for him, he notices a particular smell coming off of her. Dahl uses imagery in this moment to help readers smell the odor alongside Billy, as seen in the following passage:
Now and again, he caught a whiff of a peculiar smell that seemed to emanate directly from her person. It was not in the least unpleasant, and it reminded him—well, he wasn't quite sure what it reminded him of. Pickled walnuts? New leather? Or was it the corridors of a hospital?
Here the narrator notes how Billy “caught a whiff of a peculiar smell,” noting that it reminded him of “pickled walnuts,” “new leather,” or “the corridors of a hospital.” All of these descriptions are meant to engage readers' olfactory sense as well as to hint to them that something is amiss. As readers learn later in the story, the landlady has spiked Billy’s tea with something (likely cyanide), but he isn’t able to deduce what it is because he’s still filtering the smell through his preconceptions of the landlady. Walnuts, leather, and hospitals are all much more benign than poison and are things he’s more likely to associate with a feeble older woman. This is one of the many moments in which Billy’s naivety—along with the landlady’s deceiving appearance—keep him from understanding the danger he is in.