It's a sparkling August day on Washington Island and the resonant notes of early classical music float on the breeze toward the sailboats and ferries that ply the waters of Death's Door strait. After a forty-year absence, the Viola da Gamba Music Festival has returned to the picturesque isle on the tip of Wisconsin's Door County peninsula. Sheriff Dave Cubiak enjoys a rare day off as tourists and a documentary film crew hover around the musicians.   The jubilant mood sours when an unidentified passenger is found dead on a ferry. Longtime residents recall with dismay the disastrous festival decades earlier, when another woman died and a valuable sixteenth-century instrument - the fabled yellow viol - vanished, never to be found.   Cubiak follows a trail of murder, kidnapping, and false identity that leads back to the calamitous night of the twin tragedies. With the lives of those he holds most dear in peril, the sheriff pursues a ruthless killer into the stormy northern reaches of Lake Michigan.