Northanger Abbey begins with a "biographical notice" of Jane Austen's life written by Henry Austen, one of Jane's older brothers, after Jane Austen's death. The notice gives some details of her life and describes her character. It also quotes from several of Austen's letters. It paints a picture of an elegant, mature, and happy woman.
The advertisement, written by Austen herself before her death, tells the reader that the novel was originally written in 1803, and asks them to remember that the book is more than thirteen years old and "places, manners, books, and opinions have undergone considerable changes" since that time.
Henry Austen's biographical notice of Jane is similar to a eulogy. The notice gives Austen's true name, rather than maintaining her anonymity. Her true name was used after her death. The notice gives information about Austen's life and character and also describes some of the hardships she went through, particularly in the sale and publication of her novels. The biographical notice has little to do with the actual novels it prefaces, but it sets up an image of the author that can only play on the reader's sympathies. It is interesting that this notice (which was used to preface a four-volume set of both Northanger Abbey and Persuasion) was written by Henry, the namesake of the character Henry Tilney in Northanger Abbey. Henry was one of Austen's favorite brothers.
It seems likely that Austen wrote the advertisement because she was concerned that, in the thirteen years that had passed since she had completed Northanger Abbey, many of the "places, manners, books, and opinions" had changed, making her book dated. Since Austen's books were generally published shortly after she finished them, they had always seemed timely and up-to-date to her readers—the fashions and the topics of conversation were all things her readers would have seen and talked about recently. But since Northanger Abbey was older, Austen felt her readers should be warned, lest they think she wrote an outdated book.