Introduction to Probability by Charles M. Grinstead & J. Laurie Snell is a free textbook that may be a good reference for the probability material we are covering. It is available online as a pdf at https://math.dartmouth.edu/~prob/prob/prob.pdf, or you can read it online on LibreTexts (like OpenStax, LibreTexts is a platform for free textbooks in a variety of subjects).

I suggest starting with Sec 1.2 (Discrete Probability Distributions), which starts with the basic definitions we have been discussing: sample spaces, properties of probability, tree diagrams, etc.