Let's just cut to the chase, shall we?
As far as the Simon Necronomicon is concerned, you'd be hard pressed to prove that it's actually an ancient magical text. The fact of the matter is that there is no record of a Necronomicon existing prior to H.P. Lovecraft's invention, and the manuscript that Simon says he received from some rare book thieves is nowhere to be found.
Yes, it's just that cheesy paperback that's been in print for over three decades now. Just about everyone who bought Anton LaVey's Satanic Bible has this, and it's good for spicing up the occasional RPG session, but not much more.