The Yellow Paper was created by Gavin Wood as a formal specification of the Ethereum protocol sketched by Vitalik Buterin in the Ethereum White Paper a few months earlier.
Some people view the Yellow Paper as brilliant work while others find it very math-syntax heavy and unreadable.
Here is the current version and a snapshot of the first release which was published on Gavin Wood's website.
The Github repository was also created on 1st April 2014.
The Yellow Paper has not been updated since 2023, and has been superceded by a Python-based alternative.