•Why it earns a slot
Lady Susan is Austen's only sustained experiment with a charismatic, unredeemed female antagonist as central consciousness, and its epistolary structure lets readers watch the same social performance described approvingly by the schemer and with horror by her observers, making it a compact study in dramatic irony and the gap between self-presentation and reality.
Told entirely through letters, Lady Susan follows the widowed Lady Susan Vernon, a brilliant and utterly unscrupulous schemer, as she maneuvers through polite society seeking a wealthy second husband while attempting to force her timid daughter Frederica into a mercenary marriage. She nearly ensnares the young Reginald De Courcy despite the warnings of his suspicious sister, but is ultimately exposed when Mrs. Mainwaring reveals her ongoing affair to Reginald. The novella ends with Lady Susan marrying the foolish Sir James Martin herself, Frederica finding refuge with the Vernons, and Reginald left to recover and eventually, it is hoped, turn his affections toward Frederica.
This distillation is written from the freely available original, which is always the better read when you have the time: gutenberg.org.