Many people can only name one occult book: the Necronomicon. Everything about it screams "occult" in the Hollywood sense of the term: dark magic driving men insane, invocations of dark and dangerous powers, ancient history, dangerous knowledge, even the term "necro" in itself name, implying dealing with the dead.
That's a pretty impressive level of fame, particularly since the Necronomicon has never actually existed.
Origins of Idea of the Necronomicon
Howard Philips Lovecraft, commonly known as H.P. Lovecraft, was a horror writer in the early 20th century. His stories make multiple references to the Necronomicon, some even giving page numbers where the information can supposedly be found!
Other horror writers who were friends of Lovecraft also started referencing the Necronomicon, as well other magical texts both real and imaginary. The fact that you can find some of these books in real life adds to the illusion that all of these books really do exist, even though they don't.
Since the 1970s, several different authors have published books entitles The Necronomicon, and have claimed their product to be ancient.
The most commonly known one is the Simon Necronomicon, which has sold hundreds of thousands of copies and has remained in print for 30 years. The introduction of the book is written by a man identifying himself only "Simon," who claims that he came into possession of a historical text written by Abdul Alhazred the Mad Arab who has been cursed by his knowledge of this material.
This version of the Necronomicon draws significantly from Sumerian and Babylonian mythology, mentioning beings like Tiamat and Marduk. Many of the incantations are even done in Babylonian, further reinforcing the illusion of the Necronomicon being rooted in reality.
Validity to the Claims?
Over the years, Simon has published a handful of books associated with the Necronomicon while claiming the information has ancient origins. However, other people who worked on the publication of the Simon Necronomicon have stated that the book was a hoax, created in part because the occult shop which originally offered the book (the Magickal Childe) kept getting requests for the Necronomicon as if it was, in fact, real.
The confusion over the validity of the Necronomicon continues with such strength that Daniel Harms and John Wisdom Gonce III Put out a book entitled The Necronomicon Files: The Truth Behind The Legend systematically addressing the historical problems of accepting any of the Necronomicons as real.
Still, there are still those who accept the book as truth. One argument for it, for example, insists that the book is written in code so only true magicians will understand the context. Unfortunately, this delves into the realm of conspiracy theory, where evidence to the contrary is considered more evidence for the secretive nature of the topic. Some attribute it to Aleister Crowley. As for how Lovecraft himself came to know of the book, some suggest that the content came to him through supernatural means. For evidence, they point to the fact the Lovecraft stated that the name for the tome came to him in a dream.
Fictional yet Useful Tool
Moreover, there are magical practitioners who accept that the book is not factually true but still find it to be a useful framework for magical workings. Many magicians consider the practitioner to be the main source of power and change in magical workings. Thus, symbols and words have as much meaning as the magician gives them.