Deep Learning, Ian Goodfellow, Yoshua Bengio, and Aaron Courville, 2016 (MIT Press) - A comprehensive textbook offering a thorough mathematical and theoretical introduction to neural networks, including detailed explanations of forward propagation, linear transformations, and vectorized operations.
Neural Networks and Deep Learning, Michael A. Nielsen, 2015 (Determination Press) - An accessible online book that clearly explains the fundamental concepts of neural networks, including the mechanics of weighted sums and bias in a neuron. Ideal for beginners.