i actually read through the simon necronomicon and i can say, its about 90% horseshit. Aprox 8.2% of it is historical info and aprox 1.8% is a mix of lovecraftian lore and bad historical assumptions.

I say this because the majority of the deities it lists are Babylonian and not Sumerian. the enuma Elish was a Babylonian text and not a Sumerian text. In the sumerian creation myth, Enlil was the major deity and not Marduk. And in either culture, Tiamat was never treated as evil. She was neutral, it was only that she was put in a bad situation(having to choose between her children and her husband) and she could not decide. And Leviathan did not evolve from Tiamat. Leviathan was a separate entity.
And it lists that Sumeria is a place. there is no such thing as Sumeria. The capitol of the sumerian empire was the city Sumer.

the sumerian creation myth was far less complex then the babyloonian one. It could be summarized as:
gods where hungry
gods where lazy
gods made man to serve them
not a very glorious start for us, is it?

Though some of this could be attributed to the fact that at the time that Simon wrote that book, there was little available information on sumerian mythology. And that he just filled in the gaps with Babylonian. Plus there was no accurate way to translate Sumerian cuneiform until the late 80s. But those facts in no way help out the credibility of the Simon necronomicon.