Get very good with the instruction set. Get very good at gdb. Then learn about memory management and networking protocols. Look up networking libraries in C.
To start, I'd say have a good C manual (K&R), have a good book for computer systems (Computer Systems by Randy Bryant, but there are other good ones) and a good documentation on gdb. gdb is where it begins.
Expert C Programming is a good book to have around because it discuses some of the more esoteric corners of the C standard. These are the places that you'll end up exploring.
To start, I'd say have a good C manual (K&R), have a good book for computer systems (Computer Systems by Randy Bryant, but there are other good ones) and a good documentation on gdb. gdb is where it begins.