[xeyownt]

The best recommendation is "Git Internals" (https://github.com/pluralsight/git-internals-pdf). It teaches you how git works from the internal, and give you absolute confidence and understanding on how the tool works.

I guess it'd take one day of your life to read it, but I think it pays back a lot.

[dreamcompiler]

This was indeed one of the sources I read to become a git expert. Git is simple and elegant on the inside. Which you'd never believe if you only studied its UI.