Yesterday this guest had rung the bell and demanded a partner to play lawn tennis -- as if the hotel kept partners laid away in drawers like so many sheets. The guest, Jerymn Hilliard, Jr., is unexpectedly at loose ends in tiny Valedolmo. He has seen all the Baedeker sights already . . . Then he learns that in a rose-colored villa, surrounded with cypress and with a terrazzo on the lake, lives another American -- one with a daughter: a Signorina Constantina: "beautiful like the angels in Paradise," says Gustavo.American novelist Jean Webster, best known for Daddy-Long-Legs, which became a silent picture for Mary Pickford, also wrote popular books for young people, including this entertaining story of an American abroad.