For the last, hmm, 6 months I\'ve been reading into Programming in C, I got myself K&Rv2, BEEJ\'s socket guide, Expert C programming, Linux Systems Programming, the ISO/IEC
If you're interested in security, try coding a web application with security in mind. If you create something like a forum you'll not only learn about a number of security concerns, but also how to use multiple technologies like databases, web servers, and a new language and potentially new style besides C. And then when you're done with your project, compare the code to other similar software. If you get stuck on your project you can also look at similar projects to see what they do. And it can be as simple or as complicated as you want it to be.
Web programming may be different from systems programming in a lot of ways, but both share many security concerns.