ANDREW MORTON is one of the world's best-known biographers and a leading authority on modern celebrity. His groundbreaking 1992 biography revealed the secret world of Princess Diana, prompting Tina Brown to declare in "The Diana Chronicles," "The journalist Morton most reminds me of is Bob Woodward." "Diana: Her True Story" became a #1 "New York Times" bestseller, as did "Monica's Story," Morton's portrait of the young woman behind the blue dress in the Clinton White House. The winner of numerous awards, including Author of the Year by the British Book Awards and Scoop of the Year by the Londo