I read the K&R book cover to cover and would not say I have any great understanding of C. Some time doing the exercises in K&R would be hugely beneficial.
I'm sure C libraries would make you more productive writing programs, but if it is simply learning C you are interested in, then you can implement anything yourself that you need. www.projecteuler.net is a good source of problems (although slightly mathematical in general) for you to get started on, if you fancy trying some coding outside of the K^R exercises.
