One Thing Well demonstrates how one command (git checkout), while it does do 'one thing well' (changing your working directory to match something in the index), can handle a wide variety of seemingly unrelated use cases.
Only the Gods demonstrates how history can mean different things: commit parentage, which is usually immutable, and branch history, which is ephemeral.
The Hobgoblin probably refers to the Emerson quote "A foolish consistency is the hobgoblin of little minds."
The Long and the Short of It is about how git accepts `git --help command`, `git command --help`, and `git command -h` but `git -h command` returns an argument error.
A theme here is that git does not try to anticipate your workflow. Its commands and options are named based on what they do, rather than how you should probably use them. This makes git a very powerful tool, but with an interface that is often counter-intuitive.