I just finished the digitalis chapter -- a fairly fast read, since for once this was one dealing with stuff of which I had at least a working knowledge going in. Christie herself discusses some of the basics re: digitalis in Appointment with Death and some of her short stories (most notably, The Herb of Death), and more importantly, one of the key characters in Dorothy L. Sayers's The Unpleasantness at the Bellona Club is a doctor -- the very same doctor who prescribed digitalis to his fellow club member, the victim, for his heart condition, in fact -- and he discusses the workings of digitalis with Lord Peter Wimsey in some detail after they've both "viewed" the body. Sayers was as obsessed as Christie about getting the chemistry of her novels involving poison right (she even co-wrote The Documents in the Case with a chemist, Robert Eustace, and they performed lab tests together to make sure the murder could really have been carried out the way they were, um, plotting it). It's obvious that she'd read up on digitalis as well.
Hmm, The Unpleasantness at the Bellona Club was published in 1928, and the RL case that Harkup thinks may have inspired Christie -- the Marie Becker murders -- happened in 1932. Mme. Becker, for her part, wouldn't happen to have been inspired by Sayers, would she?! At any rate, I'm fairly certain that Sayers was aware of the other case that Harkup mentions (Pommerais / Pauw); though the facts are not identical, there are certain elements of that case that also show up in The Unpleasantness at the Bellona Club.