Quake 2 Source Code Review
See: Quake 2 Source Code Review -- Fabien Sanglard
Fabien Sanglard has written an amazing review on the insight that the game company id had when developing Quake 2 -- not from the perspective of a gamer but down to the nuts and bolts of program design. It is especially interesting for me because I used to hear about many of these features when the game first came out, before I knew anything about programming.
In fact, games like Quake 2 were the stimulus that got me interested in programming in the first place. Hearing about the cryptic-but-astonishing news that John Carmack had broken some previous limit that everyone thought was the ceiling of what games could be. Actually, even the basics of matrix transformations floored me at the beginning.
Ah, those where the good days.