X Window System Basics (2014)
I continue to be amazed that X is still around.  I can sort of understand why it was done the way it was done back when it was first created (although even then, the concept of running the actual code remotely but the X server locally seems strange).  There <i>must</i> be more efficient ways to organize a graphics library now; it seems like X continues to be the Linux standard from inertia.