To begin with, this is the NECRONOMICON published by Chaosium as part of their Mythos fiction line. It is, of course, a fictional work. This anthology does not purport to be anything BUT fiction, so if you are looking for a real grimoire of ancient evil, yeah, good luck with that.
Second, this is absolutely huge. There are 5 "Necronomicons" included, plus some pseudo-Necronomica added as commentary. I agree with previous posters that this is overkill. And truth be known, you will probably do as I did and read the first two and skip the rest, so chronology trumps merit in this case. I am surprised, though, that any reviewers complained about this "feature" - surely more material for the same price is a good thing, right? I am happy knowing that if I ever need a Necronomicon, I have 5 to choose from (or perhaps the reviewers are worried about a "Nine Gates"-esque dilemma?).
The fiction section of THE NECRONOMICON is quite enjoyable, beginning with Manly Wade Wellman's "The Parchment" and ending with Fed Chappell's "The Adder". These stories cover 185 pages, which would make a respectable book on their own. My favorite is by far "Settler's Wall", which is the mental equivalent of living in a world of rational numbers and then running into the number "pi".
Finally, THE NECRONOMICON opens and closes with pieces by editor Robert Price. I have labored through enough editor's introductions and story notes expounding his theories of higher criticism and his religious opinions that I have finally cracked and decided to become his arch-nemesis. However, I feel I must give him credit where it is due for his materful introduction discussing the Necronomicon, postmodernism, higher-criticism, and holy scriptures. Never before have I read such a clear and obvious testament of a cultist who has studied arcane texts to the point that his brains have turned to cottage cheese and run out his ears. Really, I think I was driven insane halfway through his twenty page postmodernist critique of the existence of the concept "book" (fortunately, the next ten pages drove me further to the point of being sane again. Who knew the mind is a moebius strip?). The scary thing is, that I'm not sure if Price meant it as a satire, a fictional account by a crazed cultist, or if he really believes this stuff? I think the ambiguity only adds to the genius.
So, congratulations Robert Price, you have compiled an outstanding anthology. And, if I may say so, you'd make a dam fine cultist.