A lively, informative ode to comets co-written by popular science's most versatile guru. Words like ""yesteryear,"" last heard in the prologue to The Lone Ranger, live on in Sagan's prose: He speaks of comets breaking ""the gravitational shackles that had bound them to the Sun, and. . .now liberated from their long servitude, embarking on odysseys into the vast spaces between the stars."" Nonetheless, as the book progresses, Sagan and co. gradually unfold a rich array of unusual data--facts, theories, and anecdotes not available in the scads of other recent books tied to Halley's comet. Peppered with tags from Byron, Shelley, Juvenal, and even from Dr. Johnson (""If at your coming princes disappear,/Comets! Come every day--and stay a year!""), the book offers much that is both amusing and stimulating. The focus is not so much on Halley's itself as on humanity's long-term relationship with comets. Sagan and Druyan explore the recent arguments that the collision of earth with a comet caused the changes in climate that spelled extinction for the dinosaurs; further, they speculate that traumatic collisions have occurred more recently. Other events convincingly related to comets include Montezuma's surrender to Cortez--and the outrageous theory of contemporary astronomer Fred Hoyle that comets do bring pestilence: he believes that many outerspace particles (of which comets are partially composed) are bacteria. The exposition is occasionally deceptive. When the authors discuss the old story about the Pope who excommunicated a comet during the Dark Ages, it at first appears that they believe it--only in their conclusion do they deflate the legend. The evident strategy is to whip up interest by presenting a story in its most vivid form, and then to apply the skeptical brakes--objective scientific criteria. Readers who don't mind this intellectual tickle and slap will find much diversion here.