Over 56,000 historical books and other documents, most published prior to 1923, are available online for downloading or browsing at Project Gutenberg (https://www.gutenberg.org/), with more being added all the time. A quick search of the term “astronomy” yields the following:
The Discovery of a World in the Moone: Or, A Discovrse Tending To Prove That ‘Tis Probable There May Be Another Habitable World In That Planet (1638)
John Wilkins (1614-1672)
The Study of Astronomy, Adapted to the capacities of youth (1796)
John Gabriel Stedman (1744-1797)
The Martyrs of Science, or, The lives of Galileo, Tycho Brahe, and Kepler (1841)
David Brewster (1781-1868)
Lectures on Astronomy (1854)
The Wit and Humor of America, Volume V. (1911)
George Horatio Derby (1823-1861), writing under the name of John Phoenix
Marshall Pinckney Wilder (1859-1915), editor
Letters on Astronomy: In which the Elements of the Science are Familiarly Explained in Connection with Biographical Sketches of the Most Eminent Astronomers (1855)
Denison Olmsted (1791-1859)
The Uses of Astronomy: An Oration Delivered at Albany on the 28th of July, 1856 (1856)
Edward Everett (1794-1865)
Cosmos: A Sketch of the Physical Description of the Universe, Vol. 1 (1858)
Alexander von Humboldt (1769-1859)
Curiosities of Science, Past and Present: A Book for Old and Young (1858)
John Timbs (1801-1875)
Astronomy for Young Australians (1866)
James Bonwick (1817-1906)
Meteoric astronomy: A treatise on shooting-stars, fire-balls, and aerolites (1867)
Daniel Kirkwood (1814-1895)
Popular Books on Natural Science: For Practical Use in Every Household, for Readers of All Classes (1869)
Aaron David Bernstein (1812-1884)
Half-hours with the Telescope: Being a Popular Guide to the Use of the Telescope as a Means of Amusement and Instruction (1873)
Richard Anthony Proctor (1837-1888)
Astronomical Myths: Based on Flammarions’s “History of the Heavens” (1877)
John Frederick Blake (1839-1906)
Camille Flammarion (1842-1925)
New and Original Theories of the Great Physical Forces (1878)
Henry Raymond Rogers (1822-1901)
Recreations in Astronomy: With Directions for Practical Experiments and Telescopic Work (1879)
Henry White Warren (1831-1912)
The Sidereal Messenger of Galileo Galilei and a Part of the Preface to Kepler’s Dioptrics Containing the Original Account of Galileo’s Astronomical Discoveries (1880)
Galileo Galilei (1564-1642)
Johannes Kepler (1571-1630)
Edward Stafford Carlos ((1842–1927), translator
Sir William Herschel: His Life and Works (1880)
Edward Singleton Holden (1846-1914)
Popular Scientific Recreations in Natural Philosophy, Astronomy, Geology, Chemistry, etc., etc., etc. (1881)
Gaston Tissandier (1843-1899)
Publications of the Astronomical Society of the Pacific, Volume 1 (1889)
Astronomical Society of the Pacific (1889-)
A Textbook of General Astronomy for Colleges and Scientific Schools (1889)
Charles Augustus Young (1834-1908)
Time and Tide: A Romance of the Moon (1889)
Robert Stawell Ball (1840-1913)
Astronomy with an Opera-glass: A Popular Introduction to the Study of the Starry Heavens with the Simplest of Optical Instruments (1890)
Garrett Putman Serviss (1851-1929)
Pioneers of Science (1893)
Sir Oliver Joseph Lodge (1851-1940)
Great Astronomers (1895)
Robert Stawell Ball (1840-1913)
The Astronomy of Milton’s ‘Paradise Lost’ (1896)
Thomas Nathaniel Orchard, M.D.
Myths and Marvels of Astronomy (1896)
Richard Anthony Proctor (1837-1888)
The Story of Eclipses (1899)
George Frederick Chambers (1841-1915)
The Tides and Kindred Phenomena in the Solar System: The Substance of Lectures Delivered in 1897 at the Lowell Institute, Boston, Massachusetts (1899)
Sir George Howard Darwin (1845-1912)
The Royal Observatory, Greenwich: A Glance at Its History and Work (1900)
Edward Walter Maunder (1851-1928)
The Story of the Heavens (1900)
Robert Stawell Ball (1840-1913)
Other Worlds: Their Nature, Possibilities and Habitability in the Light of the Latest Discoveries (1901)
Garrett Putman Serviss (1851-1929)
Pleasures of the telescope: An Illustrated Guide for Amateur Astronomers and a Popular Description of the Chief Wonders of the Heavens for General Readers (1901)
Garrett Putman Serviss (1851-1929)
A Text-Book of Astronomy (1903)
George Cary Comstock (1855-1934)
Astronomical Discovery (1904)
Herbert Hall Turner (1861-1930)
A New Astronomy (1906)
David Peck Todd (1855-1939)
New Theories in Astronomy (1906)
William Stirling (1822-1900)
Side-Lights on Astronomy and Kindred Fields of Popular Science (1906)
Simon Newcomb (1835-1909)
The Children’s Book of Stars (1907)
Geraldine Edith Mitton (1868-1955)
Mathematical Geography (1907)
Willis Ernest Johnson (1869-1951)
Astronomical Instruments and Accessories (1908)
William Gaertner and Company (1896-)
now Gaertner Scientific Corporation
The Astronomy of the Bible: An Elementary Commentary on the Astronomical References of Holy Scripture (1908)
Edward Walter Maunder (1851-1928)
A Popular History of Astronomy During the Nineteenth Century, Fourth Edition (1908)
Agnes Mary Clerke (1842-1907)
Astronomical Curiosities: Facts and Fallacies (1909)
John Ellard Gore (1845-1910)
The Future of Astronomy (1909)
Edward Charles Pickering (1846-1919)
History of Astronomy (1909)
George Forbes (1849-1936)
Astronomy for Amateurs (1910)
Camille Flammarion (1842-1925)
Astronomy of To-day: A Popular Introduction in Non-Technical Language (1910)
Cecil Goodrich Julius Dolmage (1870-1908)
The World’s Greatest Books — Volume 15 — Science (1910)
Arthur Mee (1875-1943), editor
Sir John Alexander Hammerton (1871-1949), editor
The Science of the Stars (1912)
Edward Walter Maunder (1851-1928)
Are the Planets Inhabited? (1913)
Edward Walter Maunder (1851-1928)
Woman in Science: With an Introductory Chapter on Woman’s Long Struggle for Things of the Mind (1913)
John Augustine Zahm (1851-1921), writing under the name H. J. Mozans
A Field Book of the Stars (1914)
William Tyler Olcott (1873-1936)
An Introduction to Astronomy (1916)
Forest Ray Moulton (1872-1952)
Scientific Papers by Sir George Howard Darwin. Volume V. Supplementary Volume (1916)
Sir George Howard Darwin (1845-1912)
Ernest William Brown (1866-1938), contributor
Sir Francis Darwin (1848-1925), contributor
The gradual acceptance of the Copernican theory of the universe (1917)
Dorothy Stimson (1890-1988)
Astronomical Lore in Chaucer (1919)
Florence Marie Grimm
Lectures on Stellar Statistics (1921)
Carl Vilhelm Ludwig Charlier (1862-1934)
The Star People (1921)
Gaylord Johnson
Terrestrial and Celestial Globes Volume 1: Their History and Construction Including a Consideration of their Value as Aids in the Study of Geography and Astronomy (1921)
Edward Luther Stevenson (1858-1944)
Terrestrial and Celestial Globes Volume 2: Their History and Construction Including a Consideration of their Value as Aids in the Study of Geography and Astronomy (1921)
Edward Luther Stevenson (1858-1944)
Astronomy for Young Folks (1922)
Isabel Martin Lewis (1881-1966)
Astronomy: The Science of the Heavenly Bodies (1922)
David Peck Todd (1855-1939)
The New Heavens (1922)
George Ellery Hale (1868-1938)
Watchers of the Sky (1922)
Alfred Noyes (1880-1958)
Biography of Percival Lowell (1935)
Abbott Lawrence Lowell (1856-1943)