Lady Susan is shortest and least known novel of Austen's and was written in 1794 but not published till 1875 after her death.

It is about the horrible flirt Lady Susan Vernon a widow in her thirties who has an affair with a married man. After her husband's death she stays with her brother and his family and soon enchants the handsome friendly but naïve and idealistic Reginald de Courcy who is her brother-in-law.
-The Guardian