Next time you glance at your watch and find a few minutes missing, better rush to your local hypnotist--you, too, might have been unwittingly kidnapped by a UFO. So Jacobs seems to suggest, estimating that over a million Americans have been abducted in recent years. ``We have been invaded,'' he proclaims, warning that ``the aliens have powers and technology greatly in advance of ours.'' Odd words, coming from a professor of history at Temple Univ., and a sign of the extreme oddity of the phenomenon, which Jacobs explores through extensive structural analysis. Each stage of a typical abduction, including the most sensational--the sexual encounters and reproductive experiments found in nearly all cases--are scrutinized through firsthand transcripts of hypnotic-regression interviews with more than 60 victims. The conclusion? That aliens are impregnating earth women in order to create quasi-human ``hybrids'' for some unknown purpose. The mind reels, but the sober, obviously terrified abductees make a strong case for the veracity of their experience. Jacobs's own presentation is a mixed blessing: His scholarship is punctilious, but he reveals the zeal of the converted in his debunking, after slight analysis, of all earth-bound explanations for UFO abductions (hysterical contagion, psychogenic fugue states, temporal-lobe dysfunction, and the like). By bringing solid scholarship to the pioneering efforts of Bud Hopkins and the hot prose of Whitley Strieber, this marks the next stage in UFO abduction research--and is just as likely to fly off the supermarket bookracks.