I went tonight to a talk about Cliveden, that grand country house on a hill above the Thames, where the grounds are owned by the National Trust and the house is run as a luxury hotel.   The book is by Natalie Livingstone and is about its chatelaines, beginning with the Countess of Shrewsbury in the 1660s, mistress of the Duke of Buckingham, succeeded by the Countess of Orkney, who was mistress of William III (but I thought he was gay), then Augusta of Saxe-Gotha, mistress of Prince Frederick, then the Duchess of Sutherland and Nancy Astor (she ate an apple to distract her from sex).   Christine Keeler, much the best known mistress of Cliveden, was scarcely mentioned.   Her ghost was left in the swimming pool.