Reinforcement Learning: An Introduction, Richard S. Sutton and Andrew G. Barto, 2018 (MIT Press) - Definitive textbook covering the theoretical foundations of reinforcement learning, including a detailed derivation and explanation of the Policy Gradient Theorem and its applications.
CS285: Deep Reinforcement Learning, Lecture 5: Policy Gradients I, Sergey Levine, 2020 (UC Berkeley) - Lecture notes from a deep reinforcement learning course, offering a modern perspective on policy gradient methods, their derivation, and practical considerations.