When John William Polidori, then twenty years old and fresh from medical school, accepted George Gordon, Lord Byron's invitation to accompany him as personal physician on a tour of the Continent early in 1816, he had just the opportunity he needed to launch his literary career. According to Polidori's diary, John Murray, Byron's publisher and sometime friend, had secretly offered Polidori five hundred pounds to keep a journal recording everything about the trip--a project which, if adroitly managed, could enable Polidori to appear before the avid English public not as a Byron-monger but as a fit friend for his celebrated companion. Five months later, Polidori's hypersensitivity and resentfulness led Byron to dismiss him. Yet in the meantime he had participated in the famous ghost-story contest that resulted in Mary Shelley's Frankenstein (1818) and in an unfinished story by Byron that Polidori took over and rewrote as The Vampyre (1819). This novella...