A life deemed worthy of writing a biography about is usually one that was dramatic, lived intensely, by a personality as distinctive as a Renaissance portrait or a Roman bust, and plentiful with lessons for those who confront life’s crucial challenges. At first glance, Mary Shelley’s would seem such a life. She was the paramour of poet Percy Bysshe Shelley, and later his wife; she was, by William Godwin, the daughter of a famous feminist, Mary Wollstonecraft; she wrote Frankenstein. Those were her claims to renown — and that while Shelley lived, she knew everyone worth knowing in the England of that time, the Regency, a period of voluptuous intercourse and even more prodigious eating.

After her husband’s’s untimely death, however, Mary’s life slowed. Her days subsided from the drama of her bad relations with Shelley’s family into encounters with unimportant scandals, and finally into mere existence. In British author Miranda Seymour’s account, Mary’s later life seems hardly worth living, never mind writing about. Seymour devotes as much detail to the mere-existence portion of Mrs. Shelley’s life as to the Regency period, without comment as to why: one reads page after page in which Mary moves from one rented set of rooms to another, gossips with her ever less noteworthy friends, and writes increasingly uninteresting and unsuccessful novels. The temptation to put the book down at all these points is great, but because you know that its protagonist was a famous writer of an inimitable and seminal book, you hope she will finally turn a corner and find herself famous again and surrounded by the important writers and painters of the day, with a second Frankenstein to cull from their company. It never happens. Eventually she dies; one hardly notices.

The notice due Mary Shelley had already been given. Who can not be moved by her telling of how the Frankenstein idea had come to her? She was then, in 1816, but a 19-year-old girl, living in a house on the shores of Lac Leman, with Shelley and Byron and her half-sister Claire Clairmont, with whom Byron was having an important flirtation. In the evenings they gathered; ghost stories were about. She was the last to think one up. It was, she later recalled, to be the story of the creation of a man by a man, without the help of God — an idea quite as horrific as that of Goethe’s somewhat similar Faust, then a recent literary hit.

Mary Shelley wrote Frankenstein in hardly more time than it took to think up and tell the tale of a lakeside evening. From the first, her story caught the reading public’s attention; when in 1832 it was reissued, it was already on every reader’s mind. Today, of course, even as the Frankenstein idea has metamorphosed into shock, camp, and overkill, its basic message remains: the things that we create without spiritual help eventually turn on their creators and threaten our downfall.

The creator of Frankenstein suffered no such fate. She simply faded. As Seymour takes her from year to year of her widowhood, Mary dotes on, but fails to find any spark in, her dull son Percy. She fails to win her Shelley relatives’ trust but does outlast them. She aids, avoids, and eventually reconciles with her atheistic, brilliant, bankrupt bookseller father. She tries to write a second crucial novel but fails, drawing always on formative events from years of her life ever less vividly remembered. As in deepening obscurity she eventually loses her force, health, and, suddenly, to illness, her life (she died in 1851), it’s hard not to feel how empty, how dead to the world, was every moment of Mary Shelley’s life after that torrid summer in which she wrote Frankenstein. Giving life to her great idea had buried her alive.