THREE YEARS AGO, Susan Mary Aisop had the break biographers dream of. She was asked to write the life of Victoria Sackville-West - the illegitimate daughter of a Spaninsh damoer and the British Minister to Washington a hundred years ago. When George Weidenfeld published the English edition of her first book, Letters to Marietta , in 1975, he suggested Lady Sackville as her next target. "To my utter surprise," she says, "Nigel Nicolson, Lady Sackville's grandson, immediately agreed. I went to Sissinghurst Castle where Nigel lives in a small wing, and he handed me stacks of priceless family papers which I packed into four enormous suitcases and flew to Washington."

Back at the Watergate, she stashed the papers in her hall closet and dug' in for 18 months of research. She deciphered dozens of letters and read 50 years of diaries - translating many from French and cracking the secret code that camouflaged Lady Sackville's love life. (She was involved in a series of notorious court cases, great inheritances and fasicinating friendships - including a rumoured romance with and American president.)

Lady Sackville commes out in England in June and will be published in the United States by Doubleday in September. It fills the gap between Pepita , the story of Lady Sackville's mother, and Portrait of a Marriage , the story of her daugther Vita Sackville-West's unconventional marriage to Harold Nicholson (which was written by their son, Nigel Nicholson).