The most famous magical book of occult knowledge that sounds real, but isn’t.
Possibly the most famous book that doesn’t exist, the Necronomicon is a fictional book of dark magic invented by weird fiction / horror author H.P. Lovecraft. First mentioned in 1924’s The Hound, the Necronomicon is part of Lovecraft’s Cthulhu Mythos, a dark collection of cosmic horror, ghouls, inter dimensional monsters, and unspeakable evil all set in an uncaring indifferent universe. The best interpretation of the name “necronomicon” is “book considering (or classifying) the dead”. Supposedly written in 738 CE by Abdul Alhazred (who was later eaten alive by an invisible monster in broad daylight), the Necronomicon is a dark book of forbidden knowledge and most Lovecraft characters who read it come to horrible ends.
Lovecraft felt to produce terror a story had to be “… devised with the care and verisimilitude of an actual hoax.” As such the Necronomicon is very much treated as if it were a real book. Lovecraft enjoyed making his fictional world seem believable. For example, in a list of real books he would throw in a few real-sounding fake ones (such as the Necronomicon) – blurring the line between reality and fiction. Similarly he wrote that there were copies of the Necronomicon held by 5 world institutions: the British Museum, Harvard, Bibliothèque nationale de France, University of Buenos Aires, as well as Miskatonic University … which is a fictional school set in the equally fictional city of Arkham, Massachusetts. Again, including a fictional creation in a list of real places making something fake seem real.
Part of the appeal of the Necronomicon (beyond the spooky name) is that, like all good suspenseful horror, Lovecraft gives the reader just enough details to understand the idea of the Necronomicon but the exact contents (or even a good physical description of the book) are left open to your imagination. This vagueness also kept the door open for future expansion of ideas. Soon other authors began to include the Necronomicon in their work, and so it spread.
Today the Necronomicon has gone beyond the works of Lovecraft & his friends and has appeared in countless other projects. It’s in books, movies, cartoons, comics, video games, music, etc, each with their own take on exactly what the Necronomicon is, but it’s always a book of dark magic. It’s in the The Evil Dead series, it’s in an episode of The Real Ghostbusters, Mr. Burns mentions it at a meeting of republicans in The Simpsons, it’s the name of a German thrash metal band, it’s the name of H.R. Giger’s first collection of artwork, Michael Crichton and Stephen King have both referenced it, etc. The book of the dead lives on, spreading its tentacles across dark fiction. Cthulhu fhtagn.
Added info: The fictional Arkham Asylum in the DC Universe, where many of Batman’s foes are frequently locked away, was named after the fictional Lovecraft town of Arkham, Massachusetts.