A steamy and evocative historical novel by social critic Sontag, set primarily in 18th-century Naples. The sweeping story focuses on the British Ambassador to the King in Naples, his passionate and notorious love, and her infatuation with Lord Nelson. It's all a bit much, but the history, and Naples, are very nicely evoked. (The title refers to the Ambassador's explorations of Mount Vesuvius.)  (ITL143, $16.00)