With all due respect to Bluefirephoenix…
For those who may not know it, here is a brief history of the Necronomicon:
Once upon a time there was a fiction writer called H.P. Lovecraft. He became famous for writing a series of short stories and novellas that were based on the idea that before humans, several races of Ancient Old Ones ruled the earth. These Ancient Old Ones were not friendly guys, but were immense beings of monstrous form who came from a different dimension from ours. When they ruled Earth they kept humans as slaves and cattle. For various reasons their time came and went, and they retreated into the vastness of space, or into the deep wilds of Antarctica, or beneath the darkest waters of the ocean. Any human unlucky enough to uncover traces of these ancient civilizations usually went insane or died (or both). In this story world, dark cults sprang up to help the Old Ones return…and the Old Ones did want to return, and to enslave humanity once again.
Naturally, dark cults and dark magicians who want to learn about ancient mysteries have to have some sort of book to refer to, and so for his stories H.P. Lovecraft invented the "Necronomicon", a book supposedly written by the mad Arab Abdoul Alhazrad. This book was the usual source of how Lovecraft's characters learned dark secrets, and usually learning even the smallest of these secrets drove the characters insane.
H.P. Lovecraft's stories captured people's imaginations. Lots of people were drawn to the mythos of the world he had created for his stories, and some people even believed the Necronomicon was a real book. And eventually someone actually did write a book and called it the Necronomicon, and that's the thing you find on the bookshelves of bookstores. It was (obviously) not written by a mad Arab called Abdoul Alhazrad, but it probably sold (and still sells) a lot of copies anyway. Thankfully, reading it doesn't drive people insane.
So. That's the real origin of the Necronomicon. Now, the Cthulu mythos (the general name for Lovecraft's stories) did become quite popular. So I think it's quite possible that the thoughts of all the people who read his stories and think about them might very well have created a sort of astral pocket realm that reflects the stories, and maybe that's the thing you were talking about. I suppose it might even be possible that he was astrally channeling some information from somewhere when he wrote the stories, and maybe *that's* the thing you were talking about. Either way, people haven't been thinking about this for more than a few decades, so any kind of magickal system based on it would probably still be fairly sketchy and unstable. (As opposed to, for example, Taoist magick or Kaballah magick, which people have been practicing for centuries.)
But my big question, really, (and again, all due respects to Bluefirephoenix), is…why would anyone *want* to have real truck with anything that smacks of the Cthulu mythos? As just stories they are fun, but the things Lovecraft described in his books are the kind of thing that no person who values their life or their sanity would **ever** want to really encounter. The Old Ones, as told by Lovecraft, were absolutely the antithesis of everything humans value.
I know that systems of ritual (or high/Hermetic) magick like the Kaballah, the Golden Dawn, etc. teach that there are nine dark qlippoth that mirror the nine light sephiroth (very very *very* basically, demons versus angels) and they also teach that it is necessary to face the qlippoth deliberately in order to properly control the energies of the sephiroth. But…well, just my opinion of course, but it seems to me that there might be better ways of doing that than by deliberately invoking the qlippoth, or something. I've read accounts by dark practicioners like Aleister Crowley, Anton LaVey, etc. that described how invoking forces like that can tend to obsess the people who invoke them and cause them to do all sorts of crazy things. So even if you say that systems of magick based on the Lovecraft mythos are effective (and there's no reason why they couldn't be, really)…well, I'm just not sure that using such a system is really *wise*. For sure it's something I've never wanted to tinker with.
But maybe Blue could explain that point of view a bit better?