The Englishwoman Gertrude Bell lived an extraordinary life. Her adventures are the stuff of novels: she rode with bandits; braved desert shamals; was captured by Bedouins; and sojourned in a harem. Called the most powerful woman in the British Empire, she counseled kings and prime ministers. Bell's colleagues included Lloyd George and Winston Churchill, who in 1921 invited Bell -- the only woman whose advice was sought -- to the Cairo Conference to...