Why? Because humans are silly creatures.
The Necronomicon is a fictional book and the invention of horror author H.P. Lovecraft. That wikipedia article pretty much covers it in good detail.
Now why people believe in fictional objects, I direct you to the wikipedia article about False Documents,
Two of the best known modern cases of a false document are the Fictional Texas Chain Saw Massacre film and the Fictional Blair Witch Project. Both are
entirely fictional works but there are people who swear they are depictions of actual events.
I personally think it is connected with the psychological phenomena known as True Believer Syndrome:
You can find commercial adaptions of Lovecraft's Necronomicon. The most popular and furthest away from the actual mythos is the crappy Avon books
Simon Necronomicon. It is a little black paperback book. It is pure crap, a smattering of bad Babylonian mythology, stuff ripped off of Aleister
Crowely, and barely any Mythos lore. I guess people like it becuase of the silly pentacle sigil thing on the cover. Its target audience is high school
kids who don't know any better.
The best adaption by far is Donald Tyson's version. It really does feel like an old grimoire and reads like an actual mythos tome without the sanity
Necronomicon: The Wanderings Of Alhazred by Donald Tyson (2004, ISBN 0-7387-0627-2)
You can find it here: www.amazon.com...
He also wrote some companion books that I have not read.
Alhazred: Author of the Necronomicon
Secrets of the Necronomicon
Grimoire of the Necronomicon, this one hasn't been released yet.
If your a fan of H.P. Lovercraft and his Fiction, you'll probably like Tyson's books. If your looking for some Apocalypse invoking tome that summons
Old Ones, well you won't find it. There's simply no real book like that.
If you want real books that cause real horror: I direct you to The Communist Manifesto and Mein Kampf. Now those are some real scary books.
Edit: the worst False Document in the US is not the Texas Chain Saw Massacre, its the WWE. I completely forgot about that, but there are people that
swear to the point of violence that World Wrestling Entertainment is real. Silly Humans.
[edit on 25/6/08 by MikeboydUS]