Lady Catherine, The Earl, and the Real Downton Abbey by the Countess of Carnavon
Lady Catherine seemed to have a charmed life after she married the Earl - she became the chatelaine of Highclere after her husband's father suddenly died. Even though the couple was forced to sell many possessions to pay heavy death duties, they had a wonderful social life and mixed with royalty. (Prince George was a particular favourite). They also kept lots of servants, so in some ways, this is like a story from the nineteenth century instead of the twentieth.
But Lady Catherine led a troubled life because the Earl was often away, and his philandering wore her down. She turned to alcohol and finally had a nervous collapse. She managed to pick herself up, however, divorce the Earl and have a happier life. This is probably the most interesting part of the book.
Anyone who loves "Downton Abbey" will certainly enjoy this book!