According to Howard Phillips Lovecraft the book is entirely fiction. Nevertheless, there are hoaxes available and many random references over the internet claiming that the book actually existed.
In the official website dedicated to H.P. Lovecraft there is a page that explicitly says that all versions available of the Necronomicon are hoaxes. Amongst all these things that are told about the supposed veracity of the book, there are even alleged connections with the welsh mathematician and astronomer John Dee.
It may be a silly one, but that's my skeptic question:
Is the Necronomicon historically real?
Here's a direct link to the supposedly "John Dee Version".