I've never been fascinated by celebrity for its own sake, particularly the kind where the person is born or marries into his/her fame. So I was surprised how much I enjoyed Privilege and Scandal: The Remarkable Life of Harriet Spencer, Sister of Georgiana (Janet Gleeson, 2007), especially after the cover quotes made a point of noting the Spencer sisters' distant relationship to Princess Diana. I can understand why the publisher wanted to advertise it, but I can't help rolling my eyes all the same.
Anyway. It's not that Harriet was so fascinating herself, but she lived at the epicenter of English power and either saw or was close kin to someone who saw pretty much every major event from 1780-1820. So her biography is a wonderful snapshot of public and private life during a tumultuous era.