THE BRASS RING Elizabeth couldn't take it anymore. The carnival in Venice was crowded with nothing but foolish partygoers. She'd much rather be completing her historical research in Scotland, but her aunt had insisted on her taking a break. Elizabeth fled the latest costume party and hurled herself into a waiting gondola.   But her driver was no ordinary gondolier. Sophisticated Roberto Roselli had been watching her, and he was intrigued. He'd never seen anyone spend an entire evening hiding behind a statue. Soon he was enchanted. As for Elizabeth - she never wanted this carnival madness to end.