The recent video codecs are incredibly complicated, and as a solo developer, it would be pretty hard to develop a codec by yourself. You may be able to invent some feature that improves the coding slightly, but coding efficiency is getting pretty hard to beat.
What to read and what code to learn is difficult to say depending on your objective. If your objective is getting a job in video coding, it’s not too hard cause these jobs are quite academic and you don’t get paid much, so if you know some coding and show some interest in learning the field, I don’t think it’d be too hard for you to find an intern position in a lab or something.
https://en.wikipedia.org/wiki/MPEG-1
Isn't everything relevant linked to there? If not, you'll be able to get it free here http://gen.lib.rus.ec/scimag/
Just work through this book and you are set. There is a a LOT of demand and short supply for good 3D graphics programmers. I would say it should be either Media or Graphics; both may be an overkill! Good luck.
If you want to understand the fundamentals. The best I know is this https://people.xiph.org/~xiphmont/demo/daala/demo1.shtml