GroupBy: split-apply-combine, The pandas development team, 2024 - The authoritative guide for Pandas groupby operations, explaining the concept, object iteration, and various group-wise functions.
pandas.DataFrame.groupby, The pandas development team, 2024 - The official API documentation for the groupby method, outlining its parameters, return value (the GroupBy object), and basic usage.
Python for Data Analysis, Wes McKinney, 2022 (O'Reilly Media) - A foundational text that provides comprehensive coverage of Pandas, including detailed explanations of group-by operations and iterating over groups.