Deep Learning, Ian Goodfellow, Yoshua Bengio, and Aaron Courville, 2016 (MIT Press) - A comprehensive textbook that provides a detailed theoretical foundation for deep learning, including the role and types of activation functions.
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 necessity of non-linear activation functions.