I’m certainly not aware of any place that will teach you how to read a professional gaming engine written by a pro. Other than to just look over it as long as needed. What this requires is a very good understanding of not only programming but of engine design, graphics, etc. An engine such as the Q2 engine is not an easy thing to grasp at first. What I did to understand how Quake2 worked was I started at the program’s entry point (main) and just traced code from there. When I came to a function that looked interesting or had no freaking clue what it did, I’d then go and trace the code for that function. To understand a big project like this, it’s going to take a long time of looking over the code before you start to understand what’s going on.