As a good resource for learning C I would recommend Casey Muratori's tutorials: https://www.youtube.com/watch?v=F3ntGDm6hOs&list=PLEMXAbCVnm...

If you are interested in learning more about writing complex software in C consider checking out HandmadeHero.

Sadly there need to be more good resources on learning how to write good C and low level software. I am hoping my article can be a starting point for people who wish to learn about library design for example.