Introduction to Probability, Joe Blitzstein and Jessica Hwang, 2019 (CRC Press)DOI: 10.1201/9781351217316 - A comprehensive textbook providing a solid foundation in probability, often recommended for those pursuing data science and machine learning.
Introduction to Probability and Statistics (OCW 18.05), Jeremy Orloff, Jonathan Bloom, 2014 (MIT OpenCourseWare) - MIT's free online course offering lectures and materials that thoroughly explain core probability concepts like sample spaces and events.