When Marple opens a book on psychiatry, we see that the book was taken in the library five times from 1941 to 1951. However, To Define True Madness first came out in 1953 (by Sidgwick and Jackson), and the cover shown before belongs to the revised (Penguin Books) edition in 1955. In the Agatha Christie's work (Chapter 21) neither the title of the book nor the library insert is mentioned.