I'm a big fan of git's tutorial manpages: gittutorial, gittutorial-2, and especially gitcore-tutorial which digs into the actual on-disk representation to let you understand what the upper layers are actually doing. Understanding the underlying model helps you reason about how to accomplish what you want to do.

Manpages available on your local system, or they're online here:

https://git-scm.com/docs/gittutorial

https://git-scm.com/docs/gittutorial-2

https://git-scm.com/docs/gitcore-tutorial