Bagging Predictors, Leo Breiman, 1996Machine Learning, Vol. 24 (Springer)DOI: 10.1007/BF00058655 - The seminal paper introducing the bagging ensemble method for reducing variance in predictions.
The Elements of Statistical Learning: Data Mining, Inference, and Prediction, Trevor Hastie, Robert Tibshirani, and Jerome Friedman, 2009 (Springer) - A comprehensive textbook covering statistical learning, including detailed explanations of the bias-variance tradeoff, bagging, boosting, and gradient boosting. 2nd edition.