In the hectic universe of David Mitchell’s third novel, the Cloud Atlas sextet is a musical piece for overlapping soloists; its colorful ne'er-do-well composer explains to a friend that each solo is interrupted by its successor and recontinues in turn. Then he asks: ''Revolutionary or gimmicky?'' Oh, mostly gimmicky, judging by the identically structured novel at hand, which finds six stories in search of a proper telling (a feverish and Melvillean tale of the 19th-century seas, the obscure composer’s cuckolding adventures in 1930s Belgium, the deposition of a rebellious clone in some sinister future, etc.). Mitchell's talents for riotous incident and energetic prose keep the pages turning, but ''Atlas''' disparate strands are linked only by the flimsiest of pretenses—for instance, the composer discovers a portion of the ocean tale on the book-shelves of his host. The six cylinders never function as one engine.