Maybe it was her appearance on David Letterman's show, or maybe the fact that her hubby used to be President. Whatever the reason, Barbara Bush's ultradignified (and boringly titled) Barbara Bush: A Memoir has shot up from No. 14 to No. 5 this week. Full of sharp political insights like, ''That darn Hussein, he is putting all those children at risk,'' the book-which Bush wrote herself-is currently in its third printing, with 600,000 copies now circulating throughout this great land. Weeks Fiction on List 1 Debt of Honor Tom Clancy, Putnam, $25.95 4 2 The Body Farm Patricia Cornwell, Scribner, $23 2 3 The Celestine Prophecy James Redfield,Warner,$17.95 30 4 The Gift Danielle Steel, Delacorte, $15 9 5 A Son of the Circus John Irving, Random House, $25 5 6 Nothing Lasts Forever Sidney Sheldon, William Morrow, $23 1 7 Politically Correct Bedtime Stories: Modern Tales for Our Life and Times James Finn Garner, Macmillan, $8.95 11 8 The Bridges of Madison County Robert James Waller, Warner, $16.95 108 9 The Chamber John Grisham, Doubleday, $24.95 16 10 Brothers and Sisters Bebe Moore Campbell, Putnam, $22.95 1 Nonfiction 1 Men Are From Mars, Women Are From Venus John Gray, Ph.D., HarperCollins, $23 77 2 Couplehood Paul Reiser, Bantam, $19.95 3 3 In the Kitchen with Rosie Rosie Daley, Knopf, $14.95 22 4 Embraced by the Light Betty J. Eadie, Gold Leaf Press, $16.95 69 5 Barbara Bush A Memoir Barbara Bush, Scribner, $25 2 6 The Book Of Virtues: A Treasury Of Great Moral Stories William J. Bennett, Simon & Schuster, $30 38 7 Motherless Daughters: The Legacy of Loss Hope Edelman, Addison-Wesley, $22.95 8 8 The Kennedy Women Laurence Leamer, Villard, $27.50 6 9 Brando: Son gs My Mother Taught Me Marlon Brando, Random House, $25 1 10 Midnight in the Garden of Good and Evil John Berendt, Random House, $23 28 Trade Paperbacks 1 The Shipping News E. Annie Proulx, Touchstone, $12 17 2 The 7 habits of Highly Effective People Stephen R. Covey, Fireside, $12 191 3 Gumpisms: The Wit and Wisdom of Forrest Gump Winston Groom, Pocket Books, $5 4 4 Care of the Soul Thomas Moore, HarperPerennial,$12 37 5 A History Of God Karen Armstrong, Ballantine, $14 4 6 The Road Less Traveled M. Scott Peck, M.D., Touchstone, $12 347 7 Lasher Anne Rice, Ballantine, $14 4 8 Chicken Soup for the Soul Jack Caufield and Mark Hansen,editors, Health Communications, $12 9 9 What to Expect When You're Expecting Arlene Eisenberg, Heidi E. Murkoff, and Sandee E. Hathaway, Workman, $10.95 75 10 Where Angels Walk Joan Wester Anderson, Ballantine, $10 28