Some methods for classification and analysis of multivariate observations, J. B. MacQueen, 1967Proceedings of the fifth Berkeley symposium on mathematical statistics and probability, Vol. 1 (University of California Press) - This foundational paper presents an early formal description of the K-Means algorithm.
Pattern Recognition and Machine Learning, Christopher Bishop, 2006 (Springer) - Offers a thorough explanation of K-Means, often framing it as an instance of the Expectation-Maximization (EM) algorithm.