Lady Susan by Jane Austen:
This is a very clever book indeed. Quite different from the rest of Austen's oeuvre, it is not the sort of book that you can imagine a teenager might be able to write. To conceive the character of a woman of 35ish and her use of sexual attraction and seduction for a 19 year old, as Jane was when she finished this, shows remarkable powers of observation and deduction.
