Pandas User Guide: GroupBy: split-apply-combine, The pandas development team, 2024 - The official documentation providing a comprehensive guide to groupby operations, including examples of grouping by multiple columns and using various aggregation functions.
Pandas User Guide: MultiIndex / Advanced Indexing, The pandas development team, 2024 - This section of the official documentation describes the MultiIndex object, which is the resulting index structure when grouping by multiple columns.
Python for Data Analysis, Wes McKinney, 2022 (O'Reilly Media) - A foundational book by the creator of Pandas, offering in-depth explanations and examples of data manipulation, including detailed discussions on grouping by multiple keys and hierarchical indexing.