If networking and protocols are your primary area of interest, then C is what you need for that. I think that C and python together make a powerful arsenal, and recommend you work with both at the same time, but I understand you want to dive full into C if you dive at all.

K&R will be what you need to get started, and past that, you could pick up some protocol specific information. Implementing protocols in C is common practice, and there are plenty of good ways to exercise those skills such as implementing sliding window atop UDP or the like.