If you want to learn Git from the inside out, I wrote a two-parter that aims to explain Git from the inside out, focusing on the data-structure Git uses:
https://looselytyped.com/blog/2014/08/31/gits-guts-part-i/
https://looselytyped.com/blog/2014/08/31/gits-guts-part-i/
Finally, _if_ you have an O'Reilly subscription, I am currently writing Head First Git (first four chapters are in early release). If you are not familiar with the Head First series, its a rather unique format that involves using lot of pictures to explain ideas and traditionally the books move a lot slower than most technical books. Ideas/concepts are cemented using puzzles, quizzes, crosswords.
You can see a list of the existing ones here https://www.amazon.com/Head-First-Series-Books/b?ie=UTF8&nod...
Feel free to email me if you need any more resources—I have spent a lot of time teaching Git.