Sweeping and scandalous, rich and compellingly readable, here is the first biography of Lady Harriet Spencer, ancestor of Diana, Princess of Wales, and devoted sister of Georgiana, Duchess of Devonshire. Harriet Spencer was without a doubt one of the most glamorous, influential, and notorious aristocrats of the Regency period.

The second daughter of the prestigious Spencer family, Harriet was born into wealth and privilege. Intelligent, attractive, and exceedingly eager to please, at nineteen years of age she married Frederick, Viscount Duncannon, an aloof, distant relative. Unfortunately, it was not a happy union; the only trait they shared was an unhealthy love of gambling. The marriage produced four children, yet Harriet followed in the footsteps of her older sister and began a series of illicit dalliances, including one with the prominent and charismatic playwright Richard Brinsley Sheridan. Then she met Lord Granville Leveson Gower, handsome and twelve years her junior. Their years-long affair resulted in the birth of two children, and all but consumed Harriet: concealing both pregnancies from her husband required great skill. Had the children been discovered, it surely would have resulted in divorce—which would have been disastrous.

Harriet’s life was dramatic, and the history-making events she observed were equally fascinating. She was an eyewitness to the French Revolution; she participated in both the euphoria following Nelson’s victory at Trafalgar and the outpouring of grief at his spectacular funeral; she was privy to the debauchery of the Prince Regent’s wife, Princess Caroline. She quarreled bitterly with Lord Byron when he pursued her young daughter (rumor had it...