Agatha Christie, the best-selling novelist of all time, was known for her intricate murder mysteries, often involving poison. Her expert knowledge of chemistry wasn't just research; during World War I, she worked as a nurse and later as a pharmacy dispenser (apothecary's assistant). This gave her deep insight into the preparation, dosage, and effects of various medicines and toxins. Her descriptions of thallium poisoning in the novel 'The Pale Horse' were so accurate that they were credited with saving several real-life lives; doctors and nurses who had read the book recognized the symptoms in their patients and were able to administer the correct treatment. Christie's preference for poison over guns in her books reflected her belief that poisoning required more cunning and psychological complexity, making for a better 'whodunnit.'