Pattern Recognition and Machine Learning, Christopher Bishop, 2006 (Springer) - Essential textbook covering the theoretical foundations of machine learning, including comprehensive discussions on unsupervised learning and various clustering approaches.
Data Mining: Concepts and Techniques, Jiawei Han, Micheline Kamber, and Jian Pei, 2011 (Elsevier) - A widely-used textbook on data mining, providing detailed explanations of clustering algorithms, applications, and evaluation methods.
Machine Learning (CS229) Lecture Notes, Andrew Ng, Tengyu Ma, 2023 (Stanford University) - Official lecture notes from a renowned university course on machine learning, offering clear explanations and theoretical background for unsupervised learning and clustering.