Reinforcement Learning: An Introduction, Richard S. Sutton and Andrew G. Barto, 2018 (MIT Press) - Textbook that provides extensive coverage of policies, value functions, and their mathematical definitions within the Markov Decision Process framework.
Lecture 2: Markov Decision Processes (MDPs), Emma Brunskill, 2025 (Stanford University) - Lecture notes from a university course, providing a clear explanation of MDPs, policies, and value functions, suitable for students.