there's not many books that are a good middle ground and try to look at sumer with no agenda. either they take a hardline dogmatic mainstream stance or they go all in on the "i'm not saying it was aliens but it was aliens" side. if you are truly interested in it, i suggest learning cuneiform and interpreting the primary texts yourself.  as for the "esoteric truths," it's hard to say. i know they had a lot of mythology about giant serpents and dragons. as did virtually every other ancient culture. make of that what you will. i will say this though since it is humorous. when "primitive" cultures like abos and pygmy bushmen have mythologies where they talk about aliens or people coming down from the sky, mainstream scholars say that they are literal because they are primitive and stupid. but when sumerian mythology is riddled with talking about aliens and visitors from the sky, mainstream scholars say "ummm...yeah that was just allegory. they don't ack-shually mean real aliens." so that tells me there is probably something to it.
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