OpenIntro Statistics, David M Diez, Christopher D Barr, Mine Çetinkaya-Rundel, 2019 (OpenIntro) - A widely used, free, and accessible textbook providing a solid foundation in descriptive statistics, including detailed explanations of the mean, median, and mode.
Introduction to Probability and Statistics (MIT OpenCourseWare 18.05), Jeremy Orloff, Jonathan Bloom, 2014 (MIT OpenCourseWare) - Provides university-level lecture notes and course materials from MIT on fundamental statistical concepts, including a thorough explanation of central tendency measures.