Scott Chacon wrote a book on git internals that was published by peepcode some time ago. Searching for where to buy it turned up this HN thread: https://news.ycombinator.com/item?id=7999515

Looks like peepcode was acquired, but the book was open sourced: https://github.com/pluralsight/git-internals-pdf

I'm reading through it now. It starts with the fundamental git structure and works up from there.