One of the books tonight's guest Steve Quayle has written is titled Aliens and Fallen Angels(1): The Sexual Corruption of the Human Race, in which he looks at the mythology and history surrounding the idea that supernatural beings mated with earthly women.
Another book that makes some interesting connections is Angels and Aliens(2), a provocative treatise by Keith Thompson. He writes that the word angel was derived from a Greek translation of the original Hebrew "mal'akh," which meant the "shadow side of God." Thompson documents the changing perceptions of angels over the centuries and how in the past the demonic and angelic were not necessarily considered two separate types of beings.
While Thompson chooses not to reduce "complex realms to one another"-i.e. today's aliens are the same as angels of the past, he does draw parallels between the experiences. "The beings who visited George Adamski in the California desert appeared in pleasing human form (angels of light?), whereas the Men in Black portend distinctly sinister overtones (angels of darkness?)," he notes. Thompson cites the well-known 1917 "Miracle at Fatima" case where three children were visited by a being that appeared in a bright flash of light, as being a kind of crossover episode that has both religious and ufological interpretations to it.