* Everyone is going to recommend K&R, which is fine. Find a copy of C Interfaces and Implementations as well--very good implementations with explanations of standard high-level structures.

* Read source. SQLite's source, for example, is very well written and documented, and they actually have a version to download recommended for reading (everything is in one file).

* Implement. You're going to want to try implement standard Unix commands yourself. Try writing an interpreter/compiler as well. These things give you a good grip on a wide variety of C basics and will make sure you can work well with memory management and all that.

* Assembly. This is where I would learn assembly (i.e., where I did). I don't think you need to go too far in depth, but a basic working knowledge will make debugging much, much easier.