HP Lovecraft was one of the early exponents of horror fantasy, best known for the series of works known collectively as the Cthulhu Mythos. He peppered his books with references to an occult work called The Necronomicon, and, as his fame grew, he was besieged by readers asking where they could find a copy of it. But the truth was that Lovecraft had invented the book and its title. He wrote in a letter of 1937: ‘The name Necronomicon (necros, corpse; nomos, law; eikon, image = An Image [or Picture] of the Law of the Dead) occurred to me in the course of a dream, although the etymology is perfectly sound.’ So the title came before everything else, and substituted, perfectly reasonably, for the work itself.
This is a game that many writers have played, and the history of literature is full of references to books that don’t, in fact, exist. Margaret Atwood, AS Byatt, Dorothy L Sayers, Frank Herbert, Martin Amis, Arthur Conan Doyle and many, many others have all joined in. Some of my favourite fictional titles are from Kurt Vonnegut, who, as Kilgore Trout, writes non-existent works such as The Barring-Gaffner of Bagnialto, or This Year's Masterpiece, which are usually accompanied by helpful plot summaries. Perhaps the most notorious fictional-book-inventors have been writers such as Umberto Eco and Jorge Luis Borges; naturally enough, since their writing often draws attention to literature as itself an artefact.
With the Necronomicon there was a difference, however. Other writers began to treat it as if it really did exist, quoting from the nonexistent work and even composing large sections of it; several Necronomicons were in fact later published, by hoaxers including L. Sprague De Camp and Colin Wilson.
Such is the power of a good title.
Lovecraft, H. P.: Selected Letters 1934-1937 (1976)