There are several online repositories of OER materials that can be browsed by subject.
Classical Sociological Theory and Foundations of American Sociology 2018
Introduction to Research Methods in Sociology 2020
Introduction to Sociology. July 2019. Wikibooks
Introduction to Sociology, 3rd ed. Published June 2021; web version updated January 2023.
Principles of Sociological Inquiry -- Qualitative and Quantitative Methods 2012 (Older publication date, but received and recent reviews)