Also, the Necronomicon is not necromancy. Is is supposed Sumerian practice, most frequently exploring the opening of several gateways to otherworldly entities. This, again, is not Necromancy but invocation or evocation.
I would elaborate on this point and express that the Necronomicon origins are entirely rooted within a work of fiction. There is no historical basis to the book beyond clever inclusion of vague historical facts included by the science fiction horror writer H.P. Lovecraft. This is part of what made his stories so brilliant - they involved figments of occult practice that was popular at the time so as to make the stories more believable.
The most popular version of the Necronomicon is written by a man who goes only by Simon and appeared first in the 70s. It claims to be full of ancient Sumerian lore but in truth its methods and practice have more in common with practices explored in the Golden Dawn, such as the Abramelin Operation, than much of anything that was actually born of the ancient Sumerians. Indeed, the entire introduction tries to draw connections between Aleister Crowley, H.P. Lovecraft, Babylonian Mythology, and various other faiths. It's like if people took the DaVinci Code, another work of fiction, and decided it was real.
Oh wait, people did that too.