Following the spectacular flight of Whitley Strieber's Communion, UFO books once again swoop and soar through the publishing skies. Or at least they try to. This one barely leaves the ground. Not that the raw material doesn't entrance. Beginning on New Year's Eve, 1982, and continuing up to 1986 (and beyond?), more than 5,000 people have reported seeing a mysterious triangular object ""larger than a football field"" zip through the skies above the Hudson River Valley just north of New York City. Witnesses included lawyers, engineers, housewives, mechanics, and, most spectacularly, dozens of police officers including the Chief of the Danbury, Conn., force. All sound sincere; almost all reject the official explanations of wandering blimps, or ultralight aircraft flying in formation. These witnesses seem puzzled, unsettled, even scared--and well they might, for this saucy saucer (or trivet?) behaved most devilishly--bathing cars in beams of light, turning its jolly multicolored Christmas-bulb-like lights to a ""slimey green,"" racing off to the horizon at a speed that gives physicists heart failure. Along with the sightings came reports of ""missing time""--observers who found an hour or more missing from their lives, often a signal of UFO abduction. A number of aliens popped up as well; one by the name of ""Tarso,"" who identified himself as a crewman on the giant vessel, thoughtfully phoned the investigators to warn that the earth would soon be destroyed. The troika of authors harvest a rich crop of sightings, which they present in anecdotal form with a minimum of serious analysis. The writing mostly lumbers along in low gear. The piling up of details grows tedious; this could have been half or twice as long and achieved the same effect. A fascinating episode, but too many readers will keep their eyes on their watches instead of the skies.