Comets have almost always had a bad press, says Calder in this illustrated text, derived from a BBC-TV show-to-come. Emperors tremble, astronomers have their heads cut off, Harold dies at the Battle of Hastings. . . with an occasional exception: Giotto painted a comet as the Star of Bethlehem to guide the Magi. Thus the clever Calder introduces his complete comet book, a compendium of cometomania, cometohistory, cometotheories, and cometoconjectures of what to expect when, come 1985-1986, the celebrated comet Halley returns. All this is well and amusingly done--down to a dismissal of Fred Hoyle's latest theory of cometary bringers of viruses, and a photo of cometologist Fred Whipple's car with its COMETS license plate in Cambridge, Mass. (capital of cometology). Along the way Calder details the dirty snowball theory of cometary composition, and argues for cometary origins in a voluminous assemblage of particles a light year or less away--which he calls the Ã–oo Cloud, in honor of Ernst Ã–pik and Jan Ã–ort. The cloud may represent materials left over after the formation of the planets--and could be large enough to hold 100 billion future comets. (Since comets contain very little mass, that isn't so grandiose a claim.) We learn, too, a little about brandy-drinking Halley and his sometimes testy relations with Newton; we are treated to the theory that dinosaurs came to their untimely end as a result of sun-shielding dust clouds left in the wake of a collision with an ""apollo,"" a species of asteroid that may have left its mark 65 million years ago; and we're briefed on the rules of the gravitational football, solar pressure, and solar wind that account for a comet's looks and ""apparitions""--its comings and goings. Indeed, one might venture to say that one learns a little more about comets than one cares to know. But Calder is a stylish writer. If anyone can convert a cometophobe into a cometophile, he can.