It is the story behind the new movie "The Duchess" although it apparently doesn't follow the book very closely. Because I've heard the movie is not very appropriate, and because of the rating, I was happy just to look at screen caps of the costumes. When my friend told me about this book I quickly got it from the library. It was written before the movie, and is written more like a story than a biography.
Georgiana was a very influential woman, she was actively involved in politics, and a she was a fashion icon. She was best friends with Marie Antoinette, and with the Prince Regent (of whom the Regency period is named for) Along with Marie Antoinette she popularized the white "Chemise Dresses" and these hats with the large feather.
One of the Duchess's (many) homes was the lovely Chatsworth House. You might recognize it from Pride and Prejudice (2005). It is still a part of the Devonshire estate.