[benesch]

It's easy to mistake Pro Git as some sort of official Git reference when Googling, since it's hosted at

    http://git-scm.com/book

and looks an awful lot like the online man pages (git-scm.com/docs).

And so, without an explicit recommendation, it's easy to miss out on its well-organized narrative, or its superb ninth chapter, Git Internals, which explains the "why?" of Git and is probably one's best shot at grokking Git. (See parent's sibling comments for evidence.)