OpenIntro Statistics, David M. Diez, Christopher D. Barr, and Mine Çetinkaya-Rundel, 2019 (OpenIntro) - A freely available textbook offering a comprehensive introduction to statistics, including detailed explanations of descriptive statistics and measures of variability.
Introduction to Probability and Statistics (18.05) Course Materials, Jeremy Orloff, Jonathan Bloom, 2014 (Massachusetts Institute of Technology) - Provides course materials, lectures, and assignments covering foundational concepts of probability and statistics, including descriptive measures like variance and standard deviation.