Astronomy by Various Authors (2020): OpenStax. CC BY.
Astronomy is designed to meet the scope and sequence requirements of one- or two-semester introductory astronomy courses. The book begins with relevant scientific fundamentals and progresses through an exploration of the solar system, stars, galaxies, and cosmology. The Astronomy textbook builds student understanding through the use of relevant analogies, clear and non-technical explanations, and rich illustrations. Mathematics is included in a flexible manner to meet the needs of individual instructors.
Our Solar System and Other Planetary Systems edited by Elizabeth Bell & Edith Soto (2018): College of the Canyons.
A collection of texts, including selections from Astronomy, an OpenStax resource, worksheets from University of Nebraska-Lincoln, and resources from the Astronomy Education Group. Published by College of the Canyons for their ZTC (Zero Cost Textbooks) Initiative.
Astronomy and Cosmology (Khan Academy)
Astronomy and Cosmology Bookshelf (LibreTexts)
Astronomy Courses (Coursera)
General Astronomy (WikiBooks)
Hubble25: A Quarter-Century of Discovery with the Hubble Telescope (NASA)
NASA educational materials (NASA)
Teach Astronomy (Chris Impey)
UW Astronomy Education Clearinghouse (University of Washington)