While the Book itself is fictional the Simon guy wrote one based on actual Sumerian/Babylonian Mythology and actual magickical principles,with a small dash of H.P.Lovecraft thrown in,thus the rites and spells with in it have been know to work if the person knows their poop when it comes to magick.
Now before anyone goes to blasting me let me explain how if it has actual lovecraft elements in it that it'll work( as best as I can) back in the day when Lovecraft was writing his yarns people knew it was fiction but still him and his readers put thought into such being as Kutulu a.k.a Cthulhu, and then you have people of my generation (i'm 41) and my parents generation who did not have access to the fiction magazines of Lovecrafts time ,nor the great myth maker and destroyer known as the internet ( that was just being developed by the U.S.Army during my youth)and not having neither of these things you had two generations through reading his books,playing the Role play game Call of Cthulhu,and then when the Simon necronomicon hit the shelves rushing out and scooping it up basically putting all out thought and energy in to the necronomicon doing those rituals,in it and believeing etc etc.
To the point now the Cthulhu Mythos entities are now genuine valid thoughtforms/archetypes,so what started as the products of Hr. Lovecrafts nightmares and imagination in general have taken on a life of their own so to speak and become true thoughtforms,just like the Gods,Goddesses,demons,daemons,spirits etc etc of other cultures past and present!
Thought forms are astral entities. Wikipedia defines thoughtforms as:
A thoughtform is a manifestation of mental energy, also known as a 'tulpa' in Tibetan mysticism. The concept of thoughtforms is related to the Western philosophy and practice of magick.
Thoughtforms are also known as egregores. The most famous thoughtform is Golem. The definition of Golem in Wikipedia is as follows:
In Jewish folklore, a golem (????, sometimes, as in Yiddish, pronounced goilem) is an animated being created entirely from inanimate matter. In modern Hebrew the word golem literally means "cocoon", but can also mean "fool", "silly", or even "stupid". The name appears to derive from the word gelem (???), which means "raw material".
Imaginary friends, demons, genies (islamic hawass), ghosts may be considered as thoughtforms too. In my humble opinion, I think that many Gods and Goddesses can be considered as thoughtforms too.
Below are a four part video series by Rob Cutter. He explains the creation methods of thoughtforms and he also tells his own story about it. In his experience, one night he saw his creation manifest into physical reality.