Vain, brusque, and tart of tongue, Mary Poppins first blew into the Banks home at 17 Cherry Tree Lane, London, in the self-titled book of 1934, to teach Jane, Michael, and the twins some manners. Incidentally, she also took them on a series of fantastic adventures throughout London, across the globe, and within their own home—all of which she firmly denied ("stuff and nonsense") had ever taken place. The spit-spot governess with the parrot-head umbrella was the brainchild of Australian-born P. L. (Pamela Lyndon) Travers (1906-1996), who more than once declared, "I don't write for children at all. I turn my back on them." Nevertheless, the books about Mary Poppins have been translated into 25 languages and sold tens of millions of copies; their tremendous popularity is probably attributable in equal parts to the mythic elements of Mary's character, the boundless universe she creates, and popular nostalgia for...