As a child, Diane Johnson was entranced by "The Three Musketeers," dashing 17th-century residents of the famous romantic quartier called St.-Germain-des-Pres. Now, the paperback edition of her delightful book will take even more Americans to the richly historic part of the city that has always attracted us, from Ben Franklin in the 18th-century to raffish novelist Henry Miller in the 20th. <BR>Modern St.-Germain is lively and prosperous, and fifty years ago its heady mix of jazz and existentialism defined urbane cool, but Johnson takes a longer view. "Beside the shades of Jean-Paul Sartre and Edith Piaf," she writes, "there is another crowd of resident ghosts... misty figures in plumed hats whose fortunes and passions were enacted among these beautiful, imposing buildings." From her kitchen window, she looks out on a chapel begun by Reine Margot, wife of Henri IV; nearby streets are haunted by the shades of two sinister cardinals, Mazarin and Richelieu, as well as four famed queens and at least five kings. Delacroix, Corot, Ingres, David, and Manet all lived in St.-Germain; Oscar Wilde died there; and everybody who was anybody visited sooner or later. <BR>With her delicious imagination and wry, opinionated voice, Diane Johnson makes a companionable and fascinating guide to a classic neighborhood as cosmopolitan as it is quintessentially French.