From the "Truth about the Necronomicon" at www.hplovecraft.com
"Now about the “terrible and forbidden books”—I am forced to say that most of them are purely imaginary. There never was any Abdul Alhazred or Necronomicon, for I invented these names myself." - H.P. Lovecraft
The misconception about the the Necronomicon's authenticity became even more complicated when numerous hoax editions of the book started to surface in the 70s.
The Necronomicon, aka "Al Azif" or Book of the Dead is one of the world's most famous grimoires (extremely sought after books whose very existence could be nothing but a myth). It's existance has been popularized mainly by H.P. Lovecraft's fictional works, in which he characterizes the book as the work of "The mad Arab" Abdul Alhazred, and describes it as a compendium of black magic that deals mostly with the summoning and contact with supernatural (and extremely evil) forces.
While H.P. Lovecraft's version is the most widely known one, and the one most people refer to when discussing it, it's not the only account of the book, and several other myths and versions surround it related to ancient asian and middle eastern lore. There are several accounts that predate Lovecraft's work, which are in turn pointed at by his detractors as the source of his ramblings (tough Lovecraft acknowledges his work as fiction he has stated repeatedly that the book does indeed exist). These mention most prominently a book "written by non-human hands" and whose reading causes irremediable madness among other things.
The book itself is said to be a large tome and often found on stands made of human bones in European lore, tough it is difficult to track the evolution of it's myth through the ages as it is often confused with other legends and mixed in the same bag as satanism, wicca and general occultism. The book's name nonetheless has remained an underground fixture since the early 20th century and has been picked up by several other artists such as painter H.R. Giger, who has published a series of "illustrated" Necronomicons and even motion pictures such as in "The Mummy" (vaguely) and Sam Raimi's "Evil Dead" trilogy of movies in which the book plays a major role.
As per Lovecraft's accounts few copies of the books still exist, and he names one kept by a secret society of scholars in England, another housed in a Tibetan temple and a third one buried in the archives of the public library of Buenos Aires, Argentina. These accounts are partially supported by diverse sources and other copies of the book are rumored to exist somewhere around there.... Check your attic!
The story in Necronomicon was inspired by the writings of H. P. Lovecraft.