It’s been a habbit to ID software to release the source code of their previous games and game engines as open source when time is due. That’s what happened with Doom 3 as well. Since beautiful code appears to a lot of developers it’s just a logical step to analyse the Doom 3 source code with the beauty-aspects in mind.
Now there are two very good examples of such analysis.
Source 1: http://kotaku.com/5975610/the-exceptional-beauty-of-doom-3s-source-code
Source 2: ftp://ftp.idsoftware.com/idstuff/doom3/source/CodeStyleConventions.doc
Source 3: http://fabiensanglard.net/doom3/index.php
Source 4: https://github.com/TTimo/doom3.gpl