I don't know exactly what it was that people who were analyzed by Dr John Mack (Harvard prof-psychiatry) thought they were being abducted by. But as he and others have said, "there's something there, we just don't know what it is."
Mack, and Jacque Vallee, and Allen Hynek all came to basically the same conclusion...whatever it is, it isn't extraterrestrial. Rather, if this phenomena is in fact real, it seems to be extradimensional.
I would point you to this (click on the image to look are reviews at Amazon)...