Gradient Tree Boosting (Scikit-learn documentation), scikit-learn developers, 2023 - Official documentation explaining the implementation of Gradient Boosting models in Scikit-learn and how to access feature importance scores using the feature_importances_ attribute.
The Elements of Statistical Learning: Data Mining, Inference, and Prediction, Trevor Hastie, Robert Tibshirani, and Jerome Friedman, 2009 (Springer) - A comprehensive textbook covering various statistical learning methods, including detailed discussions on decision trees, ensemble methods, and the concept of feature importance derived from impurity reduction.