Probability & Statistics for Engineers & Scientists, Ronald E. Walpole, Raymond H. Myers, Sharon L. Myers, Keying E. Ye, 2016 (Pearson Education) - This is a classic and comprehensive textbook that provides a rigorous yet accessible treatment of descriptive statistics, including the theoretical basis and calculation of variance and standard deviation.
Practical Statistics for Data Scientists: 50 Essential Concepts, Peter Bruce, Andrew Bruce, Peter Gedeck, 2020 (O'Reilly Media) - This book offers a practical, applied perspective on statistical concepts for machine learning practitioners, clearly explaining variance and standard deviation within a data science context.
Introduction to Probability and Statistics (18.05), Jeremy Orloff, Jonathan Bloom, 2014 (MIT OpenCourseWare) - A free, high-quality university course that covers fundamental statistical measures, providing video lectures, notes, and problem sets for a deeper understanding.