Doom turned 25 this month, and looking back I recommend you check these books/sites out.
Masters of Doom by David Kushner. The definitive history of iD Software. Puts together the story of iD software from their early days to Doom 3’s development. Combines interviews, news articles, and even some gossip from the time and makes one hell of an interesting story. The insight it provides to the PC games business of the 90s is fantastic. I can’t recommend this book enough to anyone, if they played shooters back in the 90s, or even are interested now, this might be the best book written about the gaming business from the mid-90s. (Console Wars is a close second)
Next is Game Engine Black Book: DOOM by FABIEN SANGLARD. This book covers the development of the Doom game engine, and even covers the development of all the ports. It starts off giving you a very in depth view of computer hardware of the era; covering the 486 DX, sound cards, computer networking, and NeXT computers. The way the engine works, and many of the game design decisions that came out of it. It’s a really fascinating read, even if you know only a little bit about programming (like me).
The Doom Bible by Tom Hall. The original design document for Doom. Famous for not being followed even a little bit by the developers. (here’s a nice write up on the bible itself).
Sigil by Jon Romero. The Official unofficial 5th episode for Doom.
Finally a list of games made with the Doom Engine. (fuck yeah Heretic!)
Also, check out Brutal