git-diff(1), Git authors, 2023 - The official documentation for the git diff command, providing a complete reference for comparing different states, including working directory, staging area, and commits.
Pro Git, Scott Chacon and Ben Straub, 2014 (Apress) - A highly regarded book on Git, offering in-depth explanations of the staging area, working directory, and the various uses of git diff for inspecting changes.