Cool! I'm not quite sure where you're at in your practice, so apologies if the following is old hat to you.
Books: Original Devil book, don't bother with the 4.4 one (http://www.amazon.com/Design-Implementation-Unix-Operating-System/dp/0201546299), Uresh Vahalia's UNIX Internals, the Lions book, Muchnick's Advanced compiler book, Patterson and Hennessy's CA:AQA, err. Probably some stuff I'm forgetting.
Projects: v8 (Google's JavaScript engine) is a beautiful program.
But mostly, practice. Practice can be more important than reading, especially if you get to practice around people who are better than you. If you have the chance to write your own operating system, you should do so. My experience with this, in Brown's CS 169, was literally life-changing.