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.
With the help of over 75 astronomers and astronomy educators, the textbook (cleverly named Astronomy,) has been adapted, expanded, and updated from earlier textbooks we three authors have written. The book is free to students in the electronic version, and can be custom printed on demand – at cost. Even more interesting, the book is open source, which means you can use it as is, or develop your own electronic version of it, selecting only the sections you teach, adding your curriculum materials, etc.
Full Link: https://openstax.org/details/astronomy