What is a Codebook?
A Markdown Notebook in Git with Code
A codebook is simply Git project repository that just contains Markdown files with personal notes and code along with it than can be quickly run from the command line to remind you how something works. Codebooks are particularly valuable because you create and maintain them using your own voice and examples.
Unix and Linux engineers and hackers have been keeping a little black book with notes on how to do specific things since the 70s. It has become something of a cultural icon. These days it is far more efficient to keep those notes in a codebook online that can be accessed from anywhere.