The Incomparable's Secret: The Regency Romance Series, #3

Lady Rosalind Fenwick has spent two Seasons as the ton's celebrated Incomparable by day, and its sharpest anonymous wit by night. No one knows that "The Looking-Glass," the column half of Mayfair reads over its morning chocolate, comes from her own pen, and no one must, since exposure would cost her far more than a fashionable reputation.
When a blackmail note threatens to name her in print, Rosalind turns to the one man she can trust to help without asking why she needs it.