A garrulous but intermittently enjoyable guide to Halley's comet, which will re-visit Earth's vicinity from fall of 1985 through spring of 1986, as it has every 75 years or so since the beginning of recorded time. Harpur, founder of the Halley's Comet Society (hence the ""official"" status of this book), is an enthusiast who offers an excess of trivial detail where tighter organization and more economy would have better served his expository purposes. (One chapter on ""The Astonishing Edmund Halley"" is followed by a separate chapter devoted entirely to the correct proununciation of Halley's name!) Harpur nonetheless divulges, albeit haphazardly, a great deal of erudition, both scientifically accurate and quaintly informative. A series of illustrated chapters discusses the worldwide panic that greeted the comet on its last visit in 1910; Earth was supposed to pass through the comet's tail, and Harpur describes both the ""comet parties"" given by wealthy New Yorkers on their rooftops and the ""comet pills"" dispensed in Haiti by canny voodoo doctors. There are also charts explaining where and when the viewing will be best next year (in Africa, Australia and Chile, in April of 1986); and Harpur includes an intriguing though long-winded discussion of ""Giotto,"" a space-probe that will be launched by European scientists to study the comet up close and personal for the first time. (Because of budget cuts, NASA is unlikely to engage in much comet-related research.) From the entymology of the word comet (from the Greek for ""long-haired"") to an anlysis of the two conflicting theories about comet-composition (the ""Dirty Snowball"" and the ""Flying Sandbank,"" respectively), the book is dense with facts and figures, though most of them could have been presented in a more digestible form. This is not one of those cut-and-paste projects that publishers whip up in order to capitalize on some fad of the moment, and it's pleasantly eccentric in its British-amateur way. Still, the coming of the comet deserves a more lucid account. Let's hope someone writes one.