You want more? Good, good—as Betruger might croon, his one good eye glazing over in a kind of hellish rapture. This isn't an exhaustive list (we don't have space for that), but it should get you started. Go look here:
http://DOOM.wikia.com/wiki/DOOM—An invaluable resource for all things DOOM.
http://www.doomworld.com/idgames/—How many mods do you need?
http://toastytech.com/DOOMa/index.html—Download the alphas and betas here.
http://classicdoom.com/doomcomp.txt—Ledmeister's exhaustive port documentation.
http://www.gamers.org/pub/idgames/lmps/tyson/weapons.html—George Bell's breakdown of DOOM weapon damage.
http://www.gamers.org/docs/FAQ/bfgfaq/—Tony Fabris's extraordinary testament to the BFG.
http://rome.ro/lee_killough/—Lee Killough's DOOM archive, including level creditation and more.
http://www.gamers.org/docs/FAQ/doomfaq/—Hank Leukart's FAQ. Worth it for the poem alone.
http://www.doomworld.com/10years/doomcomic/comic.php—The DOOM comic. It's like a comic, only with more DOOM. All together now: “Dance! Dance, bonedaddy!” etc.
http://www.gdcvault.com/play/1014627/Classic-Games-Postmortem—Hall and Romero talk shop at the 2011 Game Developers Conference.
www.idsoftware.com—It's their game. You ought to think about playing their other stuff, too.
http://rome.ro—Romero is not just an important designer but a wonderful source of history and preservationist info.
www.tomtomtom.com—HallHallHall. Go visit him.
Hugh, D. A., and B. Linaweaver. 1995. DOOM: Knee-Deep in the Dead. New York: Pocket Star Books.
Hugh, D. A., and B. Linaweaver. 1995. DOOM: Hell on Earth. New York: Pocket Star Books.
Hugh, D. A., and B. Linaweaver. 1996. DOOM: Infernal Sky. New York: Pocket Star Books.
Hugh, D. A., and B. Linaweaver. 1996. DOOM: Endgame. New York: Pocket Star Books.
Costello, M. 2008. DOOM3: Worlds on Fire. New York: Pocket Star Books.
Costello, M. 2009. DOOM3: Maelstrom. New York: Pocket Star Books.
DOOM IN BOOKS
There are chapters on DOOM in the following:
Loguidice, B., and M. Barton. 2009. Vintage Games: An Insider Look at the History of Grand Theft Auto, Super Mario, and the Most Influential Games of All Time. London: Focal Press.
Donovan, T. 2010. Replay: The History of Video Games. Lewes: Yellow Ant.
And if you read Italian, here's the only other complete book on the subject:
Bittanti, M., and S. Morris, eds. 2005. Doom: Giocare in prima persona. Milan: Costa and Nolan.