Black holes--whether they exist or not--lurk at the bottom (or top) of speculation in astrophysics today. That is the message we get from veteran New York Times science writer Walter Sullivan's compendium of insights and outlooks on cosmology. There is nothing new, he notes, to the idea that a massive star, after burning its nuclear fuel, can collapse under pressure until it achieves so great a density and gravitational force that no light can escape. LaPlace was an early theorist; and with the accumulation of data leading to the discovery of white dwarfs (stars which have collapsed but stabilized short of holery), neutron stars, and more recently, X-ray binary stars, there has been growing belief that black holes exist, and indeed that one has been found--a source known as Cyg X-1. Sullivan begins his excursion with an account of the mysterious 1908 ""fireball"" in Siberia. According to English theorist Stephen Hawking, the explosion may have been caused by a ""mini"" black hole passing through the earth--one of many minis spread through the galaxy. Sullivan's historical development concentrates on the past 30 years and the advent of radio astronomy, and balloon- and rocket-based observations, which have disclosed the existence of quasars (powerful radio energy sources presumed to be very remote), pulsars (neutron stars which are the debris of exploding supernovae), and the X-ray binaries--of which Cyg X-1 is one of the most notable. Black holes have been suspected as the basis for a variety of these and other phenomena. At this point, Sullivan's easy ambling explanations of relativity, time and space warps, and the like, turn toward the prolix and complex. Bombarded with the pros and cons of black hole existence, a reader may conclude that astronomers are science-fiction inventors par excellence. Toward the end, inevitably, come speculations on the origin of the universe, grand schemes for exploiting the energy of black holes or traveling to other universes, and questions concerning ultimate destiny: are we bound to expand forever with cool death in the offing? or abide in a modified steady state? or anticipate a new big bang? Bear with Sullivan, who, when he is good is very good--though when he is too good (conscientious to a fault), he causes the eyes to glaze.