Of course, there are many ancient and worthwhile volumes that belong in this study - not least of which might be the works of Galileo or Copernicus, were it not for the fact that their works on the subject of Astronomy are so rare, and so sought after as to have been snapped up by the rare manuscripts divisions of the world's leading universities!
However, there are still some fascinating volumes available to collect out there, if we are willing to adjust our expectations somewhat. Consider instead, Fontenelle's A Plurality of Worlds, which is in fact a small pamphlet written in the form of a discursive story between a Renaissance marquis and his friend, as they walk through a starlit garden at night. First published in 1688, this slim and unobtrusive work was actually fundamental in setting the tone for the later age of scientific enlightenment, as it expounded Copernicus's heliocentric rather than terra-centric view of the solar system, and extrapolated from there the possibility of other worlds, and other life forms like ourselves. It of course attracted the ill-wishes of the Catholic Church at the time, but the tide of scientific illuminism could not be stopped. Unfortunately, the original 1688 volume is near impossible to be found, but a very fine reprint can be found from 1929, by the Nonesuch Press, for a mere hundred dollars or so.
At the other end of the spectrum of course, we have the added revelations of recent astrophysicists, and none could be more famous than Sir Stephen Hawkins. His magnum opus is still arguably his Brief History of Time (1988, Bantam Press, New York) which changed forever the way that we regard the universe. Although very recent, this book can be highly collectible if the right edition is found. The very first edition (in attractive silver dustjacket) was originally printed as a presentation copy, before the editors realized that it was riddled with errors (one being the complete lack of a table of contents page!) as such, the copy was immediately recalled and reprinted. If one of these very early copies can be found, in very find condition, then they would be worth somewhere in the region of a thousand or more US dollars!
Fontenelle, Bernard le Bovier. A Plurality of Worlds. for R. Bentley and S. Magnes: London (1688). Second edition; Translated into English by Mr. Glanvill
Manilius, Marcus. The Five Books of M Manilius. Jacob Tonson at the Judges Head: London (1697). Full title-The Five Books of M Manilius Containing a System of the Ancient Astronomy and Astrology: Together with the Philosophy of the Stoicks
Moore, Sir Jonas. A Mathematical Compendium. for J. Phillips, H. Rhodes, and J. Taylor: London (1705). Fourth edition; Full title-A Mathematical Compendium; or, Useful Practices in Arithmetick, Geometry, and Astronomy, Geography and Navigation, Embattelling, and Quartering of Armies, Fortification and Gunnery, Gauging and Dyalling
Keill, John. Introduction to the True Astronomy or Astronomical Lectures. Bernard Lintot: London (1721). Full title-Introduction to the True Astronomy or Astronomical Lectures, Read in the Astronomical School of the University of Oxford
Huygens, Christian. Opera Reliqua and Opera Posthuma. Jansson-Waesberg: Amsterdam (1728). 2 volumes; bound in full contemporary vellum over boards
Newton, Isaac. The Mathematical Principles of Natural Philosophy. Benjamin Motte: London (1729). 2 volumes; Translated into English by Andrew Motte
Mairan, Jean Jacques D'otous de. Traite Physique et Historique de l'Aurore Bore'ale. de L'Imprimerie Royale: Paris (1733). Full title-Traite Physique et Historique de l'Aurore Bore'ale: Suite des Memoires de l'Academie Royale des Sciences
Cassini, Jacques. Tables astronomiques du soleil. de L'Imprimerie Royale: Paris (1740). Full title-Tables astronomiques du soleil, de la lune, des planetes, des etoiles fixes, et des satellites de Jupiter et de Saturne; avec l'explication and l'usage de ces memes tables; Illustrated by Simonneau
Long, Roger. Astronomy in Five Books. Cambridge (1742). 2 volumes; Handmade paper covered covers
Rutherforth, T. A System of Natural Philosophy. Printed by J. Bentham for W. Thurlbourn: Cambridge (1748). 2 volumes; Full title-A system of natural philosophy, being a course of lectures in mechanics, optics, hydrostatics, and astronomy
La Condamine, Charles Marie de. Journal du Voyage fait par Ordre du Roi a l'Equateur. de L'Imprimerie Royale: Paris (1751). 2 volumes; Full title-Journal du Voyage fait par Ordre du Roi a l'Equateur, servant d'Introduction Historique a la Mesure des Trois Premiers Degres du Meridian. -- Mesure des Trois Premiers Degres du Meridien dans l'Hemisphere Austral
Watts, I. The Knowledge of the Heavens and the Earth Made Easy. T. Longman, J. Buckland, and W. Fenner: London (1760). Sixth Edition; Full title-The knowledge of the heavens and the earth made easy, or, The first principles of astronomy and geography explain'd by the use of globes and maps: with a solution of the common problems by a plain scale and compasses, as well as by the globe
Emerson, William. A System of Astronomy. J. Nourse: London (1769). Full title-A System of Astronomy: Containing the Investigation and Demonstration of the Elements of that Science
Flamstead, John. Atlas Celeste de Flamsteed. F. G. Deschamps et l'Auteur: Paris (1776). Second Edition; Other author Jean Fortin; Full title-Atlas Celeste de Flamsteed, Approuve par l'Academie Royale des Sciences, et publie sous le Privilege de cette Compagnie
Ewing, Alexander. Practical Astronomy. Peter Hill: Edinburgh (1797). Full title-Practical Astronomy: Containing a Description of the Solar System, the Doctrine of the Sphere, the Principal Problems in Astronomy, Together with Astronomical Tables of the Sun, Moon and Primary Planets
Ferguson, James. Astronomy Explained Upon Sir Isaac Newton's Principles. Stirling and Slade: Edinburgh (1821). 2 volumes; Second Edition; Other author David Brewster
Stevenson, Dr. Edward Luther. Terrestrial and Celestial Globes. Yale University Press: New Haven (1921). 2 volumes; 1000 copies; Full title-Terrestrial and Celestial Globes: Their History and Construction, Including a Consideration of their Value as Aids in the Study of Geography and Astronomy
Barnard, Edward Emerson. A Photographic Atlas of Selected Regions of the Milky Way. Carnegie Institution: Washington, DC (1927). 2 volumes; 700 copies; Edited by Edwin B. Frost. and Mary R. Calvert