David Weatherly and Nick Redfern have both done good books on the topic.I listened to a pod cast once about them but cannot remember where. The person talking about them wrote a book about them if I remember correctly. He seemed to think they could be extraterrestrial, and also evil, he said that the people that let them into their homes, or cars had bad things happen to them. I don't think he really knew what they were. I tend to believe they could be paranormal.