A biography of Lady Diana Mosley, the dazzling Mitford sister who outraged her countrymen by marrying a British fascist leader and expressing a warm admiration for Hitler to the end of her days, has been on ice awaiting her death, which happened in mid-August, and the book will now appear from Morrow in mid-October. It was the publisher's executive editor, Claire Wachtel, who bought Diana Mosley: Mitford Beauty, British Fascist, Hitler's Angel, by British journalist and author Ann de Courcy, a year ago; the deal was made with British agent Carole Blake. De Courcy, who first met Lady Diana in Paris, where she had been living in exile, 10 years ago, became her intimate and taped many hours of conversation with her; she was also granted exclusive access to her diaries and papers, on condition that she not publish during her subject's lifetime. Lady Diana was a remarkably glamorous personality whose pro-Nazi sympathies caused her to be interned in Britain during WWII; she wrote her own autobiography 25 years ago.