In her 1831 introduction to the third edition of “Frankenstein,” the Gothic masterpiece she wrote when she was only 19, Mary Shelley tries to answer the question “frequently asked” by her readers: “How I, then a young girl, came to think of and to dilate upon so very hideous an idea?” She goes on to recall the by-now-legendary circumstances of its conception: Mary and her lover (later husband) the poet Percy Bysshe Shelley, while staying at Lake Geneva with Lord Byron and his doctor, John Polidori, were drawn one rainy night into a competition to see who could write the most terrifying ghost story—a contest Mary surely...