The K&R (1) book is the classic textbook, and it's also good as reference material. There's also the man pages. It's useful to poke around /usr/include and /usr/lib if you're on Unix, and it's also useful to refresh yourself on libc (2).
There's a ##C channel on freenode which is good for language-specific questions. Don't be scared off by the newbies - the people who answer questions usually know C pretty well.
I also wrote a packet sniffer and an IRC client in C. I uploaded them to sidoh's server. (3)
(1)
http://en.wikipedia.org/wiki/The_C_Programming_Language_%28book%29(2)
http://en.wikipedia.org/wiki/C_standard_library(3)
http://ktm.sidoh.org/code/Nota Bene: In addition, I'd strongly recommend familiarizing yourself with gdb and assembly. Also: linking, loading, and dynamic libraries. And, of course, memory management, if you're not familiar with it already.