I didn't read it all, but I glanced at it.
There are slight misunderstandings, such as human beings being temporarily confined by an "earthsuit" (body) ... in truth, God CREATED MANKIND to be body + spirit, and our bodies will be resurrected. Depending on where you go in Theology, this can be anywhere from a minor error to something that leads to even more misunderstanding.
I take greater exception to the idea that the serpent in the Garden of Eden who tempted Eve is just a random, creaturly, animal serpent, who happened to tempt/deceive Eve. That is a much greater error. Will our pet cats and goldfish try to tempt us next? Were they "fallen before the fall?" It creates all kinds of problems.
But yes, demons are spirit beings, created by God, of the class of angels, and demons are fallen through sin and rebellion (which the author also doesn't seem to want to endorse the details of).