The early nineteenth century was not a good time to be a female writer -- particularly if one was audacious enough to be a female novelist. Contemporary beliefs held that no one would be willing to read the work of a woman; the fantastic success of Mary Wollstonecraft Shelley's Frankenstein served to thoroughly disprove this theory.
Frankenstein established Shelley as a woman of letters when such a thing was believed to be a contradiction in terms; only the reputation of Madame de Stael surpassed Shelley’s in Europe. De Stael, however, was more famous for continuing to publish her works despite the fact that the Emperor Napoleon had explicitly forbade her to do so, rather than for the quality of the works themselves.
Though Frankenstein is now customarily classified as a horror story (albeit the first and purest of its kind), it is interesting to note that Shelley's contemporaries regarded it as a serious novel of ideas. It served as an illustration of many of the tenets of William Godwin's philosophy, and did more to promote his ideas than his own work ever did. The novel does not, however, subscribe to all of Godwin's precepts. It stands in explicit opposition to the idea that man can achieve perfection -- in fact, it argues that any attempt to attain perfection will ultimately end in ruin.
Frankenstein is part of the Gothic movement in literature, a form that was only just becoming popular in England at the time of its publication. The Gothic mode was a reaction against the humanistic, rationalist literature of The Age of Reason; one might say it was ushered in by the death of Keats, the English author with whom Romanticism is perhaps most closely associated. Frankenstein might be seen as a compromise between the Gothic approach and the Romantic one: it addresses serious philosophical subjects in a fantastical manner. Though it confronts recognizable human problems, it can hardly be said to take place in a recognizably natural world. Some critics have suggested that this tension between Gothic and Romantic literary modes echoes the philosophical tension that existed between herself and her husband, the Romantic poet Percy Bysshe Shelley.
As the prejudice against women writers was quite strong, Shelley determined to publish the first edition anonymously. Despite this fact, the novel's unprecedented success paved the way for some of the most prominent women writers of the nineteenth century, including George Eliot, George Sand, and the Bronte sisters. All of them owed Mary a tremendous literary debt. Without the pioneering work of Mary Wollstonecraft Shelley, a great many female authors might never have taken up their pens; they might never have felt free to exhibit dark imagination, nor to engage in philosophical reflection. Without her, and the women whose work she made possible, English literature would be unquestionably the poorer.