The Elements of Statistical Learning: Data Mining, Inference, and Prediction, Trevor Hastie, Robert Tibshirani, and Jerome Friedman, 2009 (Springer) - A comprehensive textbook covering various machine learning algorithms, including a detailed explanation of K-Nearest Neighbors, its theoretical basis, and practical aspects.
Nearest neighbor pattern classification, Thomas Cover and Peter Hart, 1967IEEE Transactions on Information Theory, Vol. 13 (IEEE)DOI: 10.1109/TIT.1967.1053964 - The seminal paper that introduced and formalized the K-Nearest Neighbors algorithm, providing its theoretical foundations.
Pattern Recognition and Machine Learning, Christopher M. Bishop, 2006 (Springer) - A widely-used textbook offering a thorough treatment of pattern recognition and machine learning concepts, including a dedicated section on the K-Nearest Neighbors algorithm.
CS229 Lecture Notes: Supervised Learning (K-Nearest Neighbors section), Stanford CS229 Course Staff, 2018 (Stanford University) - Lecture notes from Stanford University's machine learning course providing a clear introduction to K-Nearest Neighbors within the context of supervised learning.