References

Think like a Git

http://think-like-a-git.net/sections/graph-theory/directed-versus-undirected-graphs.html

HEAD vs working tree vs Index

https://stackoverflow.com/questions/3689838/whats-the-difference-between-head-working-tree-and-index-in-git

A visual Git reference

http://marklodato.github.io/visual-git-guide/index-en.html

Git reset in plain english

https://stackoverflow.com/questions/2530060/can-you-explain-what-git-reset-does-in-plain-english

Git tips you didn't know about

https://mislav.net/2010/07/git-tips/

Scott Chacon's "Git Internals" PDF

https://github.com/pluralsight/git-internals-pdf/releases/tag/v2.0

Ryan Tomayko's "The Thing About Git"

https://tomayko.com/blog/2008/the-thing-about-git