Gaston Leroux 
The Phantom of the Opera [EPUB ebook] 

The Opera ghost really existed. He was not, as was long believed, a creature of the imagination of the artists, the superstition of the managers, or a product of the absurd and impressionable brains of the young ladies of the ballet, their mothers, the box-keepers, the cloak-room attendants or the concierge. Yes, he existed in flesh and blood, although he assumed the complete appearance of a real phantom; that is to say, of a spectral shade.
When I began to ransack the archives of the National Academy of Music I was at once struck by the surprising coincidences between the phenomena ascribed to the ‘ghost’ and the most extraordinary and fantastic tragedy that ever excited the Paris upper classes; and I soon conceived the idea that this tragedy might reasonably be explained by the phenomena in question.