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You're not. It's really good. Comprehensive and clear.

I wish more projects had an 'official' reference that was as good as git's.

Nope. Pro Git is an excellent book and resource on Git. It's completely worth reading it for its Git Objects chapter alone.

Nope, it's awesome and this same topic is clearly covered under the plumbing and porcelain section:

https://git-scm.com/book/en/v2/Git-Internals-Plumbing-and-Po...

Yes, the book is excellent and also recommended :) But for me, having an understanding of git's internals really helped provide a foundation for the rest. Otherwise a lot of the stuff (like rebases, or amend commits...) were just black magic.