Deep Learning, Ian Goodfellow, Yoshua Bengio, and Aaron Courville, 2016 (MIT Press) - A foundational textbook providing a comprehensive mathematical and conceptual explanation of backpropagation within deep feedforward networks.
Neural Networks and Deep Learning, Michael Nielsen, 2019 (Determination Press) - An online book offering a very clear, step-by-step derivation of the backpropagation algorithm, accessible for new learners.