Cass, the unstable female lead in Banville's two-character tango, spaces out a lot. She freezes at her first lunch with our narrator, a wizened Scrooge of great renown who notices that her sudden inertia ''brought a small shock to the air between us, as when the refrigerator, that has been throbbing to itself unnoticed, all at once falls silent, with a lurch.'' Shroud shocks its reader like that too; it throbs along powerfully enough, until out of nowhere a beautiful sentence stops things cold. Banville's prose is thick and sumptuous. His story -- following the narrator to Turin, where Cass threatens to reveal a secret he's kept since WWII -- is of secondary concern.