arXiv - https://arxiv.org/
NASA/ADS - https://ui.adsabs.harvard.edu/
https://www.astronomynotes.com/ - This introductory astronomy textbook was initially written for a course at Bakersfield College, a community college in southern California. As a one-semester general education class it was primarily intended for non-science majors, so this textbook has applicability for anyone interested in an introduction to astronomy.