OpenIntro Statistics, David Diez, Mine Çetinkaya-Rundel, and Christopher D Barr, 2019 (OpenIntro) - A free, openly licensed textbook for introductory statistics, covering hypothesis testing foundations.
Introduction to Probability and Statistics, Jeremy Orloff, Jonathan Bloom, 2014 (MIT OpenCourseWare) - Free course materials from MIT providing a structured approach to probability and statistical inference, including hypothesis testing.