Regarding the Necronomicon

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I am a little confused about this Grimoire. I've searched a lot of places, and this is apparently a fictional Grimoire.
However, its constantly referred to in other works. On top of that in EA.Koetting's book, Evoking Eternity -- he explains how one of his first evocations was from the Necronomicon.

Can someone explain to me what exactly this is?