pandas.Series.value_counts, Wes McKinney and the pandas development team, 2024 - Official documentation for the Pandas value_counts() method, explaining its usage and parameters for frequency analysis of Series.
An Introduction to Statistical Learning: With Applications in Python, Gareth James, John H. Harland Dean, Daniela Witten, Trevor Hastie, Rob Tibshirani, Jonathan Taylor, 2023 (Springer) - Provides a robust statistical perspective on descriptive analysis, including methods for summarizing and understanding categorical variables.
OpenIntro Statistics, David Diez, Mine Çetinkaya-Bartoni, Christopher Barr, 2022 (OpenIntro) - An accessible open-source textbook that covers fundamental statistical concepts, including the summarization of categorical data through frequency counts.