I enjoyed Mastering Algorithms With C. After going through that book I was able to actually get things done. I recommend this because it walks through the implementation of basic data structures that you will be working with no matter what you're doing.

K&R is a good reference, but I only found it useful after I knew enough to actually get things done. I didn't find it good to learn with, and I was already proficient in a couple of scripting languages.

I also recommend Richard Stevens books. Advanced Unix Programming and Unix Network Programming are excellent. I'm not sure what you want to work on, so that might not be relevant.

For studying how non-trivial things are implemented I like TCP/IP Illustrated Volume 2, and The Design and Implementation of the Freebsd Operating System.

One of the things you're going to find about C that's different from higher level languages is that it's so minimal that you're going to have to build up a personal library of code to get things done.

For code samples, I like to look at OpenBSD. It's all in one place and there's less cruft than most of the alternatives. This is useful because while the key tasks of a particular program might be described well in one of the books I mentioned, it's not the same as a real modern program. I've found it really helpful to actually look at something that's stood the test of time for inspiration. Also, it's man pages are awesome. I look at them often even when writing code on other operating systems.

And remember, valgrind is your friend.