Note: University-level introductions to gender and/or feminism include Beasley’s What is Feminism, Anyway?, Connell and Pearse’s Gender: In World Perspective, Finlayson’s An Introduction to Feminism, Holmes’ What is Gender?, Launius and Hassel’s Threshold Concepts in Women’s and Gender Studies: Ways of Seeing, Thinking, and Knowing, Naples’ Companion to Women’s and Gender Studies, Richardson and Robinson’s Introducing Gender and Women’s Studies, Risman et al.’s Handbook of the Sociology of Gender (among the books below), Tong’s Feminist Thought, and Wharton’s The Sociology of Gender.
Also see other introductions and overviews here. The books Feminism and Reclaiming the F Word are particularly accessible introductions.
Also see the full-text chapter-length introductions to the sociology of gender.