Pattern Recognition and Machine Learning, Christopher M. Bishop, 2006 (Springer) - Classic textbook providing a comprehensive theoretical foundation for machine learning, including detailed explanations of overfitting and underfitting in early chapters.
Deep Learning, Ian Goodfellow, Yoshua Bengio, and Aaron Courville, 2016 (MIT Press) - A foundational textbook in deep learning that provides a rigorous theoretical treatment of machine learning fundamentals, including model capacity, overfitting, and underfitting.